Blue-emitting porous silicon(PS) prepared by hydrothermal etching was treated by rapid thermal oxidation (RTO) process.Accompanying with the blue shift of the whole photoluminescence(PL) spectrum, the previous single PL peak split into two separate PL peaks. Simultaneously, the size-reduction and size-separation of the silicon nanocrystallites were observed. This result indicates that the short-wavelength emissions in PS are also strongly size-dependent.