We study the electronic properties of Cr- and W-doped single-layer MoS2 using an ab initio method according to the density functional theory. Our calculated results show the energy band structures of MoS2 are significantly affected by Cr doping, but not by W doping at a high doping concentration. The effects of Cr doping manifest as the transition of energy band structure from direct to indirect, and the decrease of band gap. Our further analysis reveals that strain is the direct reason for the change of electronic structure in the Cr-doped MoS2.