A thermally evaporated silicon monoxide (SiO) film has been experimented as the gate dielectric in graphene field-effect transistors (GFETs) due to its room-temperature and low-damage deposition without introducing chemical gases or ionized particles as in other film deposition techniques, which may cause damage to graphene. In order to evaluate the dielectric properties, a double-gated GFET was fabricated with a standard commercial thermally grown SiO2 layer as the bottom gate dielectric and thermally evaporated SiO as the top dielectric. The electrical characterizations revealed that the top-gate carrier mobility was 1081.3 cm2/Vs, reasonably comparable to the bottom-gate mobility. Furthermore, the breakdown strength of the SiO film reached 5.7 MV/cm, which was lower than that of the SiO2 dielectric (~10 MV/cm) but in the same order of magnitude. The breakdown mechanism of the SiO film was studied, and the current-voltage characteristics were in agreement with the Frenkel-Poole emission model. Finally, the relative dielectric constant of SiO was found to be 5.3, significantly higher than that of SiO2 (3.9). These results indicate that the thermally evaporated SiO can function as an excellent dielectric for graphene-based devices.