In this paper, we report the positive bias temperature instability (PBTI) effects of the back-gated MoS2 n-MOSFET with Al2O3 gate dielectric. Multilayer MoS2 was used and all measurements are carried in vacuum to avoid the electric signal contamination by the top MoS2 surface water or oxygen molecules absorption. In the stress phase, the Id-Vg curve shifts to the positive gate bias direction. In the recovery phase, the Id-Vg shifts back completely to the fresh device curve. This indicates that voltage shift is purely due to trapping or detrapping of the oxide border traps at the MoS2/Al2O3 interface and no new traps are generated during the stress time. When applying different stress voltage and measuring the Id-Vg shift, we can calculate the oxide border trap energy density Ξ(E), which has a peak value of 6×1013cm-2eV-1 at 0.035eV above EC of MoS2 conduction band.