In this paper, a near-ideal subthreshold swing MoS2 back-gate transistor with an optimized ultrathin HfO2 dielectric layer is reported with detailed physical and electrical characteristics analyses. Ultrathin (10 nm) HfO2 films created by atomic-layer deposition (ALD) at a low temperature with rapid-thermal annealing (RTA) at different temperatures from 200 °C to 800 °C have a great effect on the electrical characteristics, such as the subthreshold swing (SS), on-to-off current (I ON/I OFF) ratio, etc, of the MoS2 devices. Physical examinations are performed, including x-ray diffraction, atomic force microscopy, and electrical experiments of metal-oxide-semiconductor capacitance-voltage. The results demonstrate a strong correlation between the HfO2 dielectric RTA temperature and the film characteristics, such as film density, crystallization degree, grain size and surface states, inducing a variation in the electrical parameters, such as the leakage, D it, equivalent oxide thickness, SS, and I ON, as well as I ON/I OFF of the MoS2 field effect transistors with the same channel materials and fabrication methods. With a balance between the crystallization degree and the surface state, the ultrathin (10 nm) HfO2 gate dielectric RTA at 500 °C is demonstrated to have the best performance with a field effect mobility of 40 cm2 V-1 s-1 and the lowest SS of 77.6 mV-1 decade, which are superior to those of the control samples at other temperatures. The excellent transistor results with an optimized industry-based HfO2 ALD and RTA process provide a promising approach for MoS2 applications into the scaling of the nanoscale CMOS process.