After i set gnu and mepla for pacakage-archives in my init.el as
below,

(package-initialize)
(setq package-archives
    '(("gnu" . "http://elpa.gnu.org/packages/")
     ("melpa" . "http://melpa.milkbox.net/packages/")))

whenever i start package-list-packages, status of mepla packages is
always new.

thanks.