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.