Enhanced electrochemical performance of LiFePO4 for Li-ion batteries has been anticipated by anion doping at the O-site rather than cation doping at the Fe-site. We report on the electrochemical performance of S-doped LiFePO4 nanoparticles synthesized by a solvothermal method using thioacetamide as a sulfur source. S-doping into the LiFePO4 matrix expands the lattice due to the larger ionic radius of S2− than that of O2−. The lattice parameters a and b increase by around 0.2% with sulfur content, while that of c remains almost unchanged with only 0.03% increase. The S-doping also contributes to the suppression of antisite defects (Fe occupying Li sites), which facilitates the easy migration of Li in the diffusion channels without blockage. Owing to these effects of S-doping, the S-doped LiFePO4 nanoparticles show enhanced electrochemical properties with a high discharge capacity of ∼113 mA h g−1 even at a high rate of 10C.