Abstract: The phases and microstructure of the differently treated Mg-Al-Ca-Ti cast magnesium alloys were investigated by X-ray diffract meter and optical microscope and their Vickers’ hardness were measured.The influence of small amount additional yttrium in Mg-Al-Ca-Ti cast magnesium and the followed heat treatment on their phases,microstructure and Vickers’ hardness were studied.The results show that with the increase of the additional yttrium,the amount of the second phase increases and with more uniform distribution,the grain size decreases and the Vickers’ hardness increase significantly.Solution treatment plus aging clearly modify the microstructure of the studied cast magnesium alloys;though have no significant effect in increasing the Vickers’ hardness.