Back-gated (BG) Multi-layer MoS2 field effect transistors (FETs) have been fabricated on SiO2/Si(P++) substrate and electrically characterized. By optimizing the fabrication process and scaling down the SiO2 thickness to 10 nm, the device exhibit excellent switching performance with a subthreshold swing of 86 mV/dec and an Ion/Ioff ratio ~107. The little hysteresis and small SS jointly suggest tiny magnitude of interface traps or attached oxidants. The noise current induced by gate leakage can affect the measured switch ratio by overwhelming the effective Ioff current defined by VDS. According to the behaviors of MoS2 FETs expressed by this work and others’, BG devices with SiO2 insulator present good performance and valuable potentials underutilized for rich applications.