I create a xcontrol in lvproject but when I try to add it to palette, I found that there is no option to add .xctl file . So how can I add the self-definined control to palette?
This KB may help: http://digital.ni.com/public.nsf/allkb/20AF261D501F8D3786257085005EAE74?OpenDocument. It gives directions on how to manually add items to palettes. If you just add the .xctl file to the Controls Panel in whatever sub-palette you want, you should be able to then find it in the palette and drop it into your front panel.
