zhu> Another problem is why I need to apply "xrdb ..." every time
zhu> when I restart the Linux system? Do I miss something?
 The modern name for that ~/.Xdefaults is ~/.Xresources. 
Huh?  It has nothing to do with modernity AFAIK.
They're just two different files with different semantics (.Xdefaults
is read by X clients, whereas .Xresources is passed to the X server
via xrdb).
        Stefan