Process parameters are one of the major factors in friction stir processing. Therefore, the effects of these parameters were investigated in fabrication of AZ91/SiO 2 composite. By keeping the rotational speed at 1250rpm, three traverse speeds (20, 40, 63mm/min) were selected on 8mm thick AZ91 magnesium alloy. The prepared grove contained 8% volume fraction nano-SiO 2 powder. Microstructural properties were investigated by means of optical microscopy (OM) and scanning electron microscopy (SEM). Results show that by increasing the traverse speed, grain size decreased significantly and hardness rose. Maximum amount of hardness achieved in this experiment was 124HV and the minimum grain size was 8μm. Also tensile properties of the specimen produced in one, two and three passes were investigated by standard tensile test. Results show that FSP improved the tensile characteristics of the as cast AZ91 alloy significantly.