A facile method of combining the defect engineering with the dielectric-screening effect is proposed to improve the electrical performance of MoS2 transistors. It is found that the carrier mobility of the transistor after the sulfur treatment on the MoS2 channel is greatly enhanced due to the reduction of the sulfur vacancies during vulcanization of MoS2. Furthermore, as compared to those transistors with HfO2 and SiO2 as the gate dielectric, the Al2O3-gate dielectric MoS2 FET shows a better electrical performance after the sulfur treatment, with a lowered subthreshold swing of 179.4 mV/dec, an increased on/off ratio of 2.11 × 106, and an enhanced carrier mobility of 64.74 cm2/V·s (about twice increase relative to the non-treated MoS2 transistor with SiO2 as the gate dielectric). These are mainly attributed to the fact that a suitable k-value gate dielectric can produce a dominant dielectric-screening effect overwhelming the phonon scattering, increasing the carrier mobility, while a larger k-value gate dielectric will enhance the phonon scattering to counteract the dielectric-screening effect, reducing the carrier mobility.