The amorphous SrTiO3 (STO) thin films prepared on Si (111) substrates by pulsed laser deposition (PLD) deposition were crystallized at different temperatures for various annealing time using rapid thermal annealing (RTA) process. Grazing incidence X-ray diffraction and atomic force microscopy were applied to investigate the crystallinity and surface morphology of annealed STO thin films. The results showed that the formation of STO crystalline phase was strongly dependent on the annealing temperature, and the increase of annealing temperature led to an improvement in the crystalline quality. However, at a certain annealing temperature, though the crystalline quality and surface morphology could be improved with increasing annealing time, the grain growth showed a saturation of grain size which was independent on the annealing time. Moreover, with increasing temperature, the saturated grain size decreased at lower temperatures while increased slightly at higher temperatures. A phenomenological assumption had been introduced to explain the experimental observations.