Objective To investigate the effect of thiourea on crystalline structure,grain sizes,morphology and corrosion resistance of Ni coatings. Methods Ni coatings were prepared on the surface of 304 stainless steel with or without thiourea. The crystalline structure,grain sizes and microscopic morphology of the coatings were characterized by XRD and SEM. The corrosion resistance of the samples was analyzed by Tafel and EIS. Results After adding thiourea, the preferred orientation of Ni coatings was changed from the <200> crystal plane to the <111> crystal planes and the average grain size of Ni coatings was reduced form 60. 2 nm to 20. 7 nm. Due to the adsorption of thiourea on the metal surface,the discharge of metal ions was blocked and the cathodic polarization was enhanced. Meanwhile,thiourea molecules were adsorbed on the active sites of the crystal growth,which effectively inhibited the crystal growth. Thus, the grain size of Ni coatings was refined. For corrosion resistance,304 stainless steel with Ni coatings was the best, followed by 304 stainless steel substrate; 304 stainless steel with nickel coatings containing Ni3 S2 was the worst. Conclusion The preferred orientation of Ni coatings was changed and the average grain size of Ni coatings was reduced by thiourea. The mechanism of thiourea influencing anti-corrosion resistance of Ni coatings was proposed. Thiourea which was adsorbed on the cathode was restored to H2 S by electrochemical reduction,then NiS was generated by the reaction of H2 S and Ni2+. Ni3 S2 was further generated by the reaction of the enrichment of Ni and NiS. With the deposition of Ni,Ni3 S2 was deposited on the surface of Ni coatings and reduced compactness of Ni passivation film. Meanwhile,due to the potential difference between Ni and Ni3 S2 , corrosion of the original cells may occur. In addition, grain refinement leaded to increased defects of grain boundaries and crystal plane, which would accelerate corrosion. Therefore,the corrosion resistance of the coatings with adding thiourea was greatly reduced.