Abstract: Conventional shot peening, micro-shot peening and conventional- and micro-shot peening were conducted on carburized and quenched heavy-load gear steel 18CrNiMo7-6, respectively, and then the effects of shot peening on the surface roughness, hardness, residual stress and scuffing load were studied. The results show that the surface hardness of the tested steel was improved by the three shot peening processes; the increase in surface hardness by the conventional- and micro-shot peening was the largest, followed by the micro-shot peening. The surface roughness of the tested steel was improved by conventional shot peening while reduced by micro-shot peening and by conventional- and micro-shot peening, and the surface roughness after micro-shot peening was the lowest. Residual compressive stresses were introduced to the tested steel surface by the three shot peening processes. The maximum residual compressive stresses by micro-shot peening and by conventional- and micro-shot peening were in surface, while that by conventional shot peening was in sub-surface. The scuffing load of the tested steel after conventional- and micro-shot peening was the largest, followed by micro-shot peening, due to the improvement of surface hardness and surface residual compressive stress and the decrease of surface roughness by micro-shot peening.