The whole chemical etching process on a P-type polycrystalline silicon substrate with resistivity 1-2Ω·cm is described. The formation mechanism of porous polycrystalline silicon(PPS) microstructure was investigated. Those how the initial pits were formed and an uniform morphology of PPS was obtained are explained. Two types of etching mechanism were characterized as defect control reaction and diffusion control reaction. The morphology formed after the isotropic acidic solution etching with different etching time and HF/HNO3concentration was compared with the effect of the same etching process after anisotropic alkaline etching. The study showed that the thickness of porous polycrystalline silicon layer with chemical acidic etching entirely depended on the existence of various types of defects.