Inner diameter of capillary has important effect on the spectrum broadening when used to restrict filamentation in high pressure gas. In this paper, the dependence of spectrum broadening on the inner diameter of the capillary was experimentally studied. The results showed that the spectrum broadening was mainly due to filamentation at low pulse energy (0.4mJ) and the guide effect of capillary at high pulse energy (1.3mJ). Between these pulse energy levels, the capillary with a smaller diameter can result in much better spectrum broadening.