The effects of anti-phase domain (APD) boundaries on the magnetic properties of Ni 2 Mn(Ga 0.5 Al 0.5 ) alloy were studied by magnetic measurements and transmission electron microscopic observations. The growth of APDs at 773K followed the square root law for annealing time. While the saturation magnetization and the Curie temperature were hardly affected by the APD size, the magnetization curve in the specimen with smaller mean APD size became harder, caused by the pinning effect of the magnetic domain walls due to the APD boundaries.