Self-phase modulated optical fringe pattern are used to study the nonlinear optical response of nanocrystalline silicon produced by photoelectron chemical etching. Irradiation time-dependent changes in the refractive index are calculated for various sizes of nanocrystallites. Fabrication of porous silicon contains silicon nano- structures has been carried out via process PECE on n-type Si wafer with >100< orientation and electrolyte solution contain Hydrofluoric acid HF concentrations of (25% HF), various laser wavelengths (532nmand442nm). The observed morphological changes using scanning electron microscopy reveals formation of silicon nanostructure. There is a strong correlation between the nonlinear optical phenomenon of silicon nanocrystallites and the nanocrystallites size distribution.