diff --git a/zero-framework.el b/zero-framework.el
index e3fd3309121025aee26edcc2fffbce4a4115d73b..0a5ee36c7bf5e24528027d7138b4691a3f33ae6d 100644
--- a/zero-framework.el
+++ b/zero-framework.el
@@ -164,13 +164,18 @@ otherwise, next single quote insert close quote")
 (defvar zero-enable-debug t
   "whether to enable debug.
 if t, `zero-debug' will output debug msg in *zero-debug* buffer")
+(defvar zero-debug-buffer-max-size 30000
+  "max characters in *zero-debug* buffer. if reached, first half data will be deleted")
 
 (defun zero-debug (string &rest objects)
   "log debug message in *zero-debug* buffer"
   (if zero-enable-debug
       (with-current-buffer (get-buffer-create "*zero-debug*")
+	(goto-char (point-max))
 	(insert (apply 'format string objects))
-	(goto-char (point-max)))))
+	(when (> (point) zero-debug-buffer-max-size)
+	  (insert "removing old data\n")
+	  (delete-region (point-min) (/ zero-debug-buffer-max-size 2))))))
 
 ;; (zero-debug "msg1\n")
 ;; (zero-debug "msg2: %s\n" "some obj")
@@ -215,7 +220,7 @@ if t, `zero-debug' will output debug msg in *zero-debug* buffer")
        ("page_number" ,(1+ zero-current-page))
        ("has_next_page" ,(or (> (length (or candidates zero-candidates)) (* zero-candidates-per-page (1+ zero-current-page))) (< zero-fetch-size (* zero-candidates-per-page (+ 2 zero-current-page)))))
        ("has_previous_page" ,(> zero-current-page 0))))
-    (zero-debug "candidates:\n  %s\n  " (s-join "\n  " candidates-on-page))
+    (zero-debug "candidates: %s\n" (s-join ", " candidates-on-page))
     (destructuring-bind (x y) (zero-get-point-position)
       (zero-panel-move x y))))
 
diff --git a/zero-pinyin.el b/zero-pinyin.el
index d0f80e552a767c7e508fa4682aaa357b6be7c310..3b4b6767438fc41893f606a048c08a7cd3f362ee 100644
--- a/zero-pinyin.el
+++ b/zero-pinyin.el
@@ -109,7 +109,7 @@
 		     (nth (+ n (* zero-candidates-per-page zero-current-page)) zero-pinyin-used-preedit-str-lengths))))
     (when candidate
       (zero-debug
-       "zero-pinyin-commit-nth-candidate n=%s candidate=%s used-len=%s zero-pinyin-pending-preedit-str=%s\n"
+       "zero-pinyin-commit-nth-candidate n=%s candidate=%s used-len=%s zero-pinyin-pending-preedit-str=%S\n"
        n candidate used-len zero-pinyin-pending-preedit-str)
       (cond
        ((null zero-pinyin-state)