The effect of Al2O3 deposition on electrical properties and ambient stability of few-layer MoS2 transistors is investigated. The deposition of Al2O3 passivation layer by atomic layer deposition method resulted in the suppression of the hysteresis in the transfer curve and the enhancement of carrier mobility due to high-kappa dielectric screening effect. In addition, the negative shift of threshold voltage (enhanced n-doping) and the metalinsulator transition at the carrier density n similar to 10(13) cm(-2) is reported. The Al2O3 passivation layer helps electrical properties of MoS2 transistor are preserved even after 11 months. Al2O3 encapsulation provides a facile method to suppress extrinsic effect on the mobility of few-layer MoS2 transistors and to ensure the ambient stability.