We investigated the effects of passivation on the electrical characteristics of molybdenum disulfide (MoS2) field effect transistors (FETs) under nitrogen, vacuum, and oxygen environments. When the MoS2 FETs were exposed to oxygen, the on-current decreased and the threshold voltage shifted in the positive gate bias direction as a result of electrons being trapped by the adsorbed oxygen at the MoS2 surface. In contrast, the electrical properties of the MoS2 FETs changed only slightly in the different environments when a passivation layer was created using polymethyl methacrylate (PMMA). Specifically, the carrier concentration of unpassivated devices was reduced to 6.5 x 10(15) cm(-2) in oxygen from 16.3 x 10(15) cm(-2) in nitrogen environment. However, in PMMA-passivated devices, the carrier concentration remained nearly unchanged in the range of 1-3 x 10(15) cm(-2) regardless of the environment. Our study suggests that surface passivation is important for MoS2-based electronic devices.