Experiments are carried out on a femtosecond laser micromachining system with the wavelength 775nm. The ablated patterns on Si (100) and Si (111) surfaces affected by femtosecond pulse laser parameters are presented. The periodic structure termed ripples on silicon surface is investigated. Two kinds of ripples on the ablated surface are observed by SEM. The whole ablation region indicates the ablated field of all amount of laser energy. The ablation region forms two circular areas. The cross section image of the ablated region is showed which is cut by FIB in the orthogonal direction of the line of the center ripples. The depth of ripples is measured. Adjusted the laser fluence as about 0.3~0.33J/cm2 and the pulse number as changeable from 1 to 50, the forming process of the ripples is observed and studied by AFM. Another experiment is done that the wave-plates are inserted into the space between the objective and sample, the result of the direction of ripples changed with the wave-plates of 1/2 or 1/4 is reviewed.