Photoacoustic spectroscopy has been used to study nanoscaled SrTiO3 powder with different annealing temperature. The photoacoustic spectra show that the absorption edge of nanoscaled SrTiO3 powder has red shift with the rise of annealing temperature,which demonstrates that the energy gap becomes narrow. The decrease of energy gap is related to the growth of grains and the reduction of lattice parameter. In addition,the absorption band appeared around 700nm is induced by the transition of the captured electrons at the additional energy level of oxigen defects. The variation of the band is strongly dependent upon the sizes of the grains.