The yield and plastic deformation behaviour of wrought Mg-alloy AZ31 sheets were investigated at elevated temperatures from ambient to 300°C. It is found that the 0.2% proof stress and ultimate strength decrease linearly with increasing temperature, and the Young’s modulus goes down in the same way. Whereas the ductility, expressed by elongation, increases with temperature, in contrast to the decrease in uniform elongation. The plastic deformation anisotropy, represented by the ratio of transverse and thickness strain of the plate sample, decreases as the temperature increases, and at about 225°C, coincidentally, reaches to unity. The activation volumes for yield stress and ultimate strength are discussed with a thermal activation law-Arrhenius Equation.