A 775nm femtosecond laser is applied to single crystal silicon by direct-write processing in air. A series of holes are drilled with 150fs duration pulses, various numbers of laser pulses and laser fluences on silicon wafer. Different laser parameters and material properties influence the size of fabricated holes. The diameter and depth of holes are gradually enlarged with the increase of laser fluence and pulse number. The periodic ripple structure on silicon surface is found and explained at the same time.