SrTiO3 thin films with various compositions have been prepared on Pt coated sapphire substrates by electron cyclotron resonance plasma chemical vapor deposition at 450 °C and 600 °C. The stoichiometric (Sr/(Sr+Ti)=0.50) films have maximum dielectric constants (εr) of 190 for the 600 °C deposition and 170 for the 450 °C deposition. The dielectric constant decreases abruptly with increasing or decreasing the Sr/(Sr+Ti) value from the stoichiometry, especially for the 450 °C deposition. The Ti-rich films include titanium suboxide (TiOx). In the Sr-rich films, excess SrO is thought to be inserted into the SrO plane and TiO2 plane in SrTiO3. In addition, SrCO3 is included in Sr-rich films. The concentration of these impurities is greater for the 450 °C deposition than for the 600 °C deposition. The leakage current density increases with increasing the Sr/(Sr+Ti) value, and the leakage current density for the 450 °C deposition is larger than for the 600 °C deposition in the Sr-rich composition region. These impurity phases may be related to the higher leakage current density. These results suggest that precise composition control in stoichiometry is necessary to deposit SrTiO3 films with superior electrical properties.