The characteristics of femtosecond laser processing for silicon are reported in this paper. Below the fluence for ablation threshold, femtosecond laser irradiation induced an amorphization of crystalline Si (c-Si). Little above the fluence for the ablation threshold, non-thermal ablation occurred. Processed areas had a negligible heat-affected zone (HAZ). The thickness of the amorphoized layer and the depth of the ablated area were related to the effective light penetration depth.