Effects of Zr contents on microstructure and mechanical properties of as-cast and in heat treated Mg-5Zn-2Al magnesium alloy were investigated by OM （optical microscope） and tensile testing machine. The results verify that grain size in as-cast and in heat treated Mg-5Zn-2Al alloy can be remarkably refined with Zr addition. The tensile strength and elongation of the as-cast and heat treated alloy are firstly increased and then decreased. With 0.6% Zr addition

the desirable grain size in the as-cast alloy can be observed

and the tensile strength and elongation reach 215MPa and 12.563%

respectively. Compared with that of as-cast alloy

the tensile strength of the heat treated alloy is greatly improved with different Zr additions. With 0.6% Zr addition

the tensile strength of the heat treated alloy reaches 249MPa.