This paper reports on the superplastic deformation behavior of the rolled commercial AZ61 magnesium alloy sheet under condition of biaxial gas-pressure forming. The disk-type sheets were deformed into the hemispherical domes at varied pressures of 0.46, 0.65, 0.8 and 1.2 MPa. The comparison between the uniaxial results and biaxial results of this work was carried out. The dynamic grain growth played a great role in the deformation mechanism. Biaxial deformation mechanism has been changed from superplastic D-L controlled GBS to D-gb controlled dislocation climb creep.