The structural, electronic and magnetic properties of intrinsic defects in monolayer MOS2 have investigated systematically by first-principles calculations based on density functional theory. The results show that S vacancy and S adsorption is the most favorable intrinsic defect under Mo-rich and S-rich conditions, respectively. Mo atom adsorbed on monolayer MoS2 and Mo atom substituted at S site induced the spin-polarization, which gives rise to total magnetic moment of 4.0 mu(B) and 2.0 mu(B), respectively. The magnetism is mainly origin from the Mo adatom and the Mo substitutional atom, respectively. For the Mo and S adsorption, the Mo atom and S atom prefer to adsorb on top of the Mo atom and the S atom, respectively. In addition, S-Mo and V-Mo can realize the n-type and p-type semiconductor behavior, respectively. These predictions are useful for the spintronic devices designs. (C) 2015 Elsevier B.V. All rights reserved.