diff --git a/client.go b/client.go
index e69679a..dc42968 100644
--- a/client.go
+++ b/client.go
@@ -15,15 +15,18 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"sync"
 	"time"
 )
 
 // Client etherscan API client
 // Clients are safe for concurrent use by multiple goroutines.
 type Client struct {
-	coon    *http.Client
-	key     string
-	baseURL string
+	coon           *http.Client
+	baseURL        string
+	keys           []string
+	keySelectMutex sync.Mutex
+	keySelectIdx   int
 
 	// Verbose when true, talks a lot
 	Verbose bool
@@ -37,12 +40,32 @@ type Client struct {
 	AfterRequest func(module, action string, param map[string]interface{}, outcome interface{}, requestErr error)
 }
 
+func (c *Client) getKey() string {
+	c.keySelectMutex.Lock()
+	defer c.keySelectMutex.Unlock()
+	c.keySelectIdx++
+	if c.keySelectIdx >= len(c.keys) {
+		c.keySelectIdx = 0
+	}
+	return c.keys[c.keySelectIdx]
+}
+
+// New initialize a new etherscan API client
+// please use pre-defined network value
+func New(network Network, APIKeys string) *Client {
+	return NewCustomized(Customization{
+		Timeout: 30 * time.Second,
+		Keys:    []string{APIKeys},
+		BaseURL: fmt.Sprintf(`https://%s.etherscan.io/api?`, network.SubDomain()),
+	})
+}
+
 // New initialize a new etherscan API client
 // please use pre-defined network value
-func New(network Network, APIKey string) *Client {
+func NewMultiKey(network Network, APIKeys []string) *Client {
 	return NewCustomized(Customization{
 		Timeout: 30 * time.Second,
-		Key:     APIKey,
+		Keys:    APIKeys,
 		BaseURL: fmt.Sprintf(`https://%s.etherscan.io/api?`, network.SubDomain()),
 	})
 }
@@ -51,8 +74,8 @@ func New(network Network, APIKey string) *Client {
 type Customization struct {
 	// Timeout for API call
 	Timeout time.Duration
-	// API key applied from Etherscan
-	Key string
+	// API keys applied from Etherscan
+	Keys []string
 	// Base URL like `https://api.etherscan.io/api?`
 	BaseURL string
 	// When true, talks a lot
@@ -83,7 +106,7 @@ func NewCustomized(config Customization) *Client {
 	}
 	return &Client{
 		coon:          httpClient,
-		key:           config.Key,
+		keys:          config.Keys,
 		baseURL:       config.BaseURL,
 		Verbose:       config.Verbose,
 		BeforeRequest: config.BeforeRequest,
@@ -117,7 +140,7 @@ func (c *Client) call(module, action string, param map[string]interface{}, outco
 		err = wrapErr(err, "http.NewRequest")
 		return
 	}
-	req.Header.Set("User-Agent", "etherscan-api(Go)")
+	req.Header.Set("User-Agent", "etherscan-api-multikey(Go)")
 	req.Header.Set("Content-Type", "application/json; charset=utf-8")
 
 	if c.Verbose {
@@ -196,7 +219,7 @@ func (c *Client) craftURL(module, action string, param map[string]interface{}) (
 	q := url.Values{
 		"module": []string{module},
 		"action": []string{action},
-		"apikey": []string{c.key},
+		"apikey": []string{c.getKey()},
 	}
 
 	for k, v := range param {
diff --git a/multi_key_test.go b/multi_key_test.go
new file mode 100644
index 0000000..4c6736c
--- /dev/null
+++ b/multi_key_test.go
@@ -0,0 +1,21 @@
+package etherscan
+
+import "testing"
+
+// TestGetKey may fail is you run test parallel
+func TestGetKey(t *testing.T) {
+	countApiKey, countBackupApiKey, k := 0, 0, ""
+	for i := 0; i < 10; i++ {
+		k = api.getKey()
+		t.Logf("key: %s", k)
+		if apiKey == k {
+			countApiKey++
+		} else if backupApiKey == k {
+			countBackupApiKey++
+		}
+	}
+	equal := countApiKey == 5 && countBackupApiKey == 5
+	if !equal {
+		t.Errorf("api.getKey not working, expected 5 for each key, got main:%d , backup %d", countApiKey, countBackupApiKey)
+	}
+}
diff --git a/setup_e2e_test.go b/setup_e2e_test.go
index 64f2702..c262e8f 100644
--- a/setup_e2e_test.go
+++ b/setup_e2e_test.go
@@ -9,12 +9,16 @@ package etherscan
 
 import (
 	"fmt"
+	"log"
 	"os"
 	"testing"
 	"time"
 )
 
-const apiKeyEnvName = "ETHERSCAN_API_KEY"
+const (
+	apiKeyEnvName       = "ETHERSCAN_API_KEY"
+	backupApiKeyEnvName = "BACKUP_ETHERSCAN_API_KEY"
+)
 
 var (
 	// api test client for many test cases
@@ -22,7 +26,8 @@ var (
 	// bucket default rate limiter
 	bucket *Bucket
 	// apiKey etherscan API key
-	apiKey string
+	apiKey       string
+	backupApiKey string
 )
 
 func init() {
@@ -30,9 +35,14 @@ func init() {
 	if apiKey == "" {
 		panic(fmt.Sprintf("API key is empty, set env variable %q with a valid API key to proceed.", apiKeyEnvName))
 	}
+	backupApiKey = os.Getenv(backupApiKeyEnvName)
+	if backupApiKey == "" {
+		log.Printf("WARN: Backup API key is empty, set env variable %q with a valid API key to proceed.", backupApiKeyEnvName)
+	}
 	bucket = NewBucket(500 * time.Millisecond)
 
 	api = New(Mainnet, apiKey)
+	api = NewMultiKey(Mainnet, []string{apiKey, backupApiKey})
 	api.Verbose = true
 	api.BeforeRequest = func(module string, action string, param map[string]interface{}) error {
 		bucket.Take()