Field effect transistors using ultrathin molybdenum disulfide (MoS₂) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS₂ devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS₂ field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS₂. Uniform encapsulation of MoS₂ transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.