Few-layer molybdenum disulfide (MoS2) has attracted a great deal of attention as a semiconductor material for electronic and optoelectronic devices. However, the presence of localized states inside the bandgap is a critical issue that must be addressed to improve the applicability of MoS2 technology. In this work, we investigated the density of states (DOS: g(E)) inside the bandgap of MoS2 FET by using a current-voltage (I-V) analysis technique with the aid of high vacuum annealing (HVA). The g(E) can be obtained by combining the trap density and surface potential (psi(S)) extracted from a consistent subthreshold current (ID-sub). The electrical performance of MoS2 FETs is strongly dependent on the inherent defects, which are closely related to the g(E) in the MoS2 active layer. By applying the proposed technique to the MoS2 FETs, we were able to successfully characterize the g(E) after stabilization of the traps by the HVA, which reduces the hysteresis distorting the intrinsic g(E). Also, the change of sulfur ions in MoS2 film before and after the HVA treatment is investigated directly by Auger electron spectroscopy analysis. The proposed technique provides a new methodology for active channel engineering of 2D channel based FETs such as MoS2, MoTe2, WSe2, and WS2 .