The focusing of a thermal atomic beam to nanometer_scale dimensions can be rea lized by a completely off_resonant standing_wave laser field, which can decrease the difficulty of atom lithography experiments. The effects of atom source on a tom focusing are analyzed using the Monte_Carlo scheme and trajectory tracing me thod, and the simulation results have shown that the effective size of the targe t has a much more important effect on feature width than beam spread and longitu dinal velocity spread. Several methods for improving experiments are presented.