Several models were presented in literature to explain the photoluminescence of porous silicon, suggesting carriers originate in Si nano-structure and recombine on the surface. None of the models have a clear justification on such carriers generation and carriers dynamics with respect to the mass conservation law, since the carriers are known to originate in one place and recombine in other. In this investigation a qualitative model, namely, the Virtual Band Model (VBM) is proposed to fill in the gabs which are raised in literature.