Abstract: The single crystal silicon wafers are irradiated by the cumulative Ti: sapphire femtosecond laser pulses in SF6, air or vacuum atmosphere. The evolution of silicon surface microstructures formed in SF6 is investigated. The one-dimensional ripple structures induced by laser pulses are formed on silicon on early stages of irradiation. With increasing number of pulses, the structures gradually evolve into two-dimensional concave-convex structures. The quasi-ordered arrays of spikes with high aspect ratio are formed on silicon after six hundred laser shots irradiation. Compared with the reported results, the spikes show the characteristics of relatively low height and nonobvious ball on their tips. It is believed that gas pressure is primarily responsible for the results. The number density