In this paper, we investigate laser ablation of silicon wafers in air and water through using Ti:sapphire femtosecond laser with central wavelength of 800 nm and pulse duration of 50 fs—24 ps at a repetition rate of 1 kHz. Unique concentric rings are observed on the sample surfaces during the laser ablation in ambient air. Formation mechanisms of the inner and outer ablative structures are attributed to the thermal melting and Coulomb explosion, respectively. The influence of laser parameters, such as the laser energy, the number of pulses and the pulse duration on the radius of the ablation ring is observed experimentally. It is found that drilling a blind hole with large aspect ratio usually requires multiple laser shots with relatively small pulse energy. In the water surroundings, femtosecond laser ablation results in porous microstructure formation in the irradiated area. However, laser pulses with picosecond duration lead to large material removal by evidently nonthermal processes. Theoretical analysis suggests that this phenomenon could be explained by the photo-mechanical stress and bubble cavitation, which are strengthened with increasing pulse duration. The critical pulse width separating the two different ablation processes in the water ambience is determined experimentally for the first time.