Microstructural, optical and dielectric properties of $SrTiO_3$ thin films were investigated. These films were deposited on the ITO glasses by rf-magnetron sputtering at room temperature and subequently heat treated in $O_2$ atmosphere at various temperatures. It has been found from X-ray diffraction patterns that as-deposited films prepared at room temperature are amorphous, while the films heat treated at temperature range of 400~$600^{circ}C$ reveal the structure of pyrochlore. On the other hand, the structure of perovskite is dominantly observed in the films heat treated at temperatures over $650^{circ}C$ in which the drastic changes of optical band gap and dielectric constant are observed. In addition, the phase transition peak is observed at $272^{circ}C$ in the films heat treated at $600^{circ}C$, while the dielectric dispersion is observed at near $310^{circ}C$ in the films heat treated at $650^{circ}C$.