Equal channel angular pressing (ECAP) is an effective thermo-mechanical process to make ultrafine grains. An investigation was carried out on the friction stir welding (FSW) of ECAPed AZ31 magnesium alloys with a thickness of 15 mm. For different process parameters, the op-timum FSW conditions of ECAPed AZ31 magnesium alloys were examined. The basic characterization of weld formation and the mechani-cal properties of the joints were discussed. The results show that the effect of welding parameters on welding quality was evident and weld-ing quality was sensitive to welding speed. Sound joints could be obtained when the welding speed was 37.5 mm/min and the rotation speed of the stir tool was 750 r/min. The maximum tensile strength (270 MPa) of FSW was 91% that of the base materials. The value of micro-hardness varied between advancing side and retreating side because of the speed field near the pin of the stir tool, which weakened the de-formed stress field. The value of microhardness of the welding zone was lower than that of the base materials. The maximum value was lo-cated near the heat-affected zone (HAZ). Remarkable ductile character was observed from the fracture morphologies of welded joints. Equal channel angular pressing (ECAP) is an effective thermo-mechanical process to make ultrafine grains. An investigation was carried out on the friction stir welding (FSW) of ECAPed AZ31 magnesium alloys with a thickness of 15 mm. The basic characterization of weld formation and the mechani-cal properties of the joints were discussed. The results show that the effect of welding parameters on welding quality was marked and weld-ing quality was sensitive to welding speed. Sound joints could be obtained when the welding speed was 37.5 mm / min and the rotation speed of the stirrer was 750 r / min. The maximum tensile strength (270 MPa) of FSW was 91% that of the base materials. The value of micro-hardness varied between advancing side and retreating side because of the speed field near the pin of the stirrer tool, which weakened the de-formed stress field. The value of microhardness of the welding zone was lower than that of the base materials. The maximum value was lo-cated near the heat-affected zone (HAZ). Remarkable ductile character was observed from the fracture morphologies of welded joints.