Porous silicon (PS) treated by water vapor annealing and vacuum annealing has been studied by positron annihilation lifetime spectroscopy and age-momentum correlation measurement. It is found that after water vapor annealing, non-radiative defects are reduced and defects dominating light source appear. These two types of defects change the lifetime and the S parameter of the positron annihilation and cause a drastic enhancement in the photoluminescence (PL) efficiency. Defects that cause the PL of PS show no obvious change after annealing at 300 ℃ in vacuum, therefore the PL of the sample is not influenced.