Biporous (micro and meso pores) nature of the silica gel powder has been studied through the characteristics of positronium (Ps) annihilation. The two distinct long lifetimes namely tau(3) and tau(4) are ascribable to ortho positronium annihilation in micro cavities within the molecular grains and inter granular mesoscopic pores respectively. In the latter type of voids, a significant fraction decays via three-photon mode. A simple physical picture of Ps annihilation in the bulk of the substance through appropriate description of the models, enables us to parameterize the size of the pores, and hence illustrates positronium annihilation (PAS) as a useful in-situ probe for micro structural study of porous substances.