Doping engineering has been an emerging topic in monolayer molybdenum disulfide (mMoS2). However, the dopants used for an n-or p-Type device and the effect of doping level are of great interests toward next-generation electronic devices. In this paper, we theoretically reveal the work function tunability of mMoS2 doped by 3d transition metals. We found that the titanium dopant forms a deep-level trap in the midgap of mMoS2 but turning into n-Type donor levels in high doping concentration due to the stronger covalent bond and the stable surface morphology, which renders it the widest work function tunability among 3d transition metals. Overall, the n-Type behavior is expected by doping with chromium, copper, scandium, and titanium, whereas nickel and zinc dopants lead to the p-Type property. The findings feature the selection of dopants for the revolutionary device and highlight the impact of doping levels from the atomistic viewpoint.