diff --git a/api/config.go b/api/config.go
index c0a191f..436f0f1 100644
--- a/api/config.go
+++ b/api/config.go
@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"net/http"
 	"os"
+	"path/filepath"
+	"strings"
 
 	"github.com/gin-gonic/gin"
 	"github.com/go-gost/core/observer/stats"
@@ -127,10 +129,11 @@ func getConfig(ctx *gin.Context) {
 
 // swagger:parameters saveConfigRequest
 type saveConfigRequest struct {
-	// output format, one of yaml|json, default is yaml.
+	// output format, one of yaml|json, default is derived from the file extension.
 	// in: query
 	Format string `form:"format" json:"format"`
-	// file path, default is gost.yaml|gost.json in current working directory.
+	// file path, default is the loaded config file, or gost.yaml|gost.json in
+	// the current working directory when no config file was loaded.
 	// in: query
 	Path string `form:"path" json:"path"`
 }
@@ -155,16 +158,31 @@ func saveConfig(ctx *gin.Context) {
 	var req saveConfigRequest
 	ctx.ShouldBindQuery(&req)
 
-	file := "gost.yaml"
-	switch req.Format {
-	case "json":
-		file = "gost.json"
-	default:
-		req.Format = "yaml"
+	file := req.Path
+	if file == "" {
+		// Default to the config file that was loaded (via -C), falling back to
+		// the current working directory when the config came from elsewhere.
+		file = config.ConfigFile()
 	}
-
-	if req.Path != "" {
-		file = req.Path
+	if file == "" {
+		file = "gost.yaml"
+		if req.Format == "json" {
+			file = "gost.json"
+		}
+	}
+	switch strings.ToLower(req.Format) {
+	case "json":
+		req.Format = "json"
+	case "yaml":
+		req.Format = "yaml"
+	default:
+		// Derive the format from the file extension so a loaded JSON config
+		// is saved back as JSON (ReadFile forces format from extension).
+		if strings.ToLower(filepath.Ext(file)) == ".json" {
+			req.Format = "json"
+		} else {
+			req.Format = "yaml"
+		}
 	}
 
 	f, err := os.Create(file)
diff --git a/config/config.go b/config/config.go
index 966ad39..9c10be9 100644
--- a/config/config.go
+++ b/config/config.go
@@ -7,6 +7,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/mitchellh/mapstructure"
@@ -28,8 +29,21 @@ func init() {
 var (
 	global    = &Config{}
 	globalMux sync.RWMutex
+
+	// configFile remembers the last file loaded via ReadFile so the API can
+	// save back to the same location (issue go-gost/x#123).
+	configFile atomic.Value // string
 )
 
+// ConfigFile returns the path of the last config file loaded via ReadFile,
+// or an empty string when the config came from stdin, a URL, or inline JSON.
+func ConfigFile() string {
+	if s, ok := configFile.Load().(string); ok {
+		return s
+	}
+	return ""
+}
+
 func Global() *Config {
 	globalMux.RLock()
 	defer globalMux.RUnlock()
@@ -780,6 +794,7 @@ func (c *Config) ReadFile(file string) error {
 	if err := v.ReadInConfig(); err != nil {
 		return err
 	}
+	configFile.Store(file)
 	return v.Unmarshal(c, viper.DecodeHook(gostDecodeHook()))
 }