In this study, the effect of sodium sulfate on the microstructure and corrosion resistance of high phosphorus Ni-P electroless coatings has been investigated. Ni-P electroless coating was deposited on AISI 1045 steel substrate with using sodium sulfate as an additive in the plating bath (with concentration of 0-1.5 g/L). Electrochemical behavior and corrosion resistance of the coatings were investigated by evaluation of open circuit potential (OCP) and potentiodynamic polarization curves. The results showed that at an optimum concentration of sodium sulfate (similar to 1 g/L), deposition rate was maximum and the size and number of microporosities in the coating decreased to the minimum values. In addition, the best corrosion resistance and the most extensive passive region in potentiodynamic polarization curve was obtained at the optimum concentration of additive. Also its OCP was the same as pure nickel.