$SrTiO_3$ thin films were deposited on Pt/Ti/$SiO_2$/Si substrates at low temperatures below $300^{circ}C$ by r.f. magnetron sputtering. The materials and the electrical properties of the deposited films were investigated with controlling deposition parameters such as substrate temperature(T_s) and positive substrate d.c. bias voltage. Stoichiometric $SrTiO_3$ films were obtained at Ts of $300^{circ}C$, but Sr content in the film was less than that of a target when Ts was lower than $300^{circ}C$, resulting in poor electrical properties. By introducing a positive substrate d.c. bias during deposition, the crystallinity and the dielectric properties of the films were markedly improved. 400 nm thick $SrTiO_3$, films deposited at $300^{circ}C$ with a positive substrate d.c. bias of 20V showed a columnar structure with <211> crystallographic direction and a dielectric constant of 98.