The electrical device performances of graphene directly grown on a SiO2 substrate have been improved through the precise adjustment of growth conditions such as growth temperature and growth time in plasma chemical vapor deposition (CVD). Only at the suitable combination of growth time and temperature, high quality and uniform graphene sheet can be directly grown on a SiO2 substrate. Forward and reverse sweeps of sourcedrain current (Ids) vs. gate bias voltage (Vgs) showed small hysteresis, possibly caused by the clean surface of the graphene device fabricated by plasma CVD, a technique that did not involve any transfer. Four-point probe measurements to evaluate the intrinsic sheet resistance of the fabricated graphene showed its value to be 170 - 200 Ω/sq, a value much lower than that of graphene directly grown on SiO2 substrate by other techniques. This low sheet resistance possibly originated from the high quality of graphene obtained by plasma CVD. These observations suggest that graphene directly grown on SiO2 substrate by plasma CVD should be a very promising candidate for fabrication of graphene-based high-performance electrical devices.