The optical properties of semiconductor nanocrystalline powder were studied by using photoacoustic spectroscopy technique. The band gap and the optical absorption coefficient of semiconductor nanocrystalline powder of TiO2, ZnO and Al-doped ZnO were measured by normalized photoacoustic spectroscopy technique. The results show that the optical properties of semiconductor nanocrystalline powder relate to particle size and particle shape. The band gap and the optical absorption coefficient of semiconductor nanocrystalline powder can be controlled by its fabricating techniques. By doping and changing the size and the shape of nanocrystals, changing the optical and electrical properties was achieved.