Photoluminescence (PL) spectra of porous silicon (PS) an fitted by a theoretical model based on quantum confinement of electrons in Si nanocrystallites having spherical and cylindrical forms. This model permits one to correlate the PL spectra with the PS structure. It was found that the PS structure is almost independent of the porosity of the PS samples when elaborated in the same HF solution, but it depends on the composition of the electrolytic solution and post-anodisation treatments such as oxidation. The specific surface area (SSA) was estimated and was found to decrease linearly when the porosity increases. it was pointed out that the SSA plays a key role in the PL behaviour within laser it-radiation. The effect of laser irradiation on the PL behaviour has been discussed according to the proposed model, and was shown to be dependent on ambient atmosphere. It was shown that the crystallite size decreases throughout photo-oxidation under laser irradiation in air.