Achieving low Schottky barriers interface is important for future electronic devices based on transition metal dichalcogenides. In this study, we demonstrate that the electronic properties of single-layer molybdenum disulfide (MoS2) can be tuned from semiconducting to metallic via controlling its defect level (sulfur vacancy) by using hydrogen plasma. Raman, Photoluminescence, Atomic Force Microscopy, and X-ray photoelectron spectra suggest that the top layer sulfur in SL-MoS2 can be totally removed and stabilized in ambient environment. Field-effect transistors (FETs) with highly sulfur vacancies/Ti electrodes fabricated and exhibited high performance n-FETs. The results provide a promising route for low Schottky barrier contacts devices with graded junction interface.