In this report

the porous silicon formed by anodization of crystal silicon wasstudied by positron annihilation technique (PAT) and scanning electron microscopy(SEM). The PAT experiments showed that the mean life and vacancy defectsincreased with the increasing anodization time. While the intensities of the longestlifetime

several ns~tens ns (orth- positronium) droped down. Small single-crystal Sispheres with mean radius of a few μm were observed by SEM after anodization. Pitswith mean radius of a few U m from the divorcement of single-crystal spheres werealso observed after further anodization. The increases of vacancy defects might bethat the extension of structures of porous silicon towards inner layer withanodization tAne and caused more vacancy defects in inner layer. Our SEMobservation presented another possibility of the increase of density of vacancy defectsin surface layer induced by the change of structures.