Effects of Sr and Ca combined additions (Sr+Ca=0.05wt.%) on the microstructure of A390 aluminum alloy were evaluated by investigating the variation of quantity, size and shape of primary silicon, eutectic silicon, primary α phase and intermetallic phase. The results showed that the variation of microstructures resulted from the synergistic effect of Sr-Ca additions. The burned value and Sr loss rate were reduced with a small amount of Ca, meanwhile, coarse effect of primary silicon phase arising from Sr addition was also suppressed. The effect of Sr in A390 aluminum alloy was mainly modifying eutectic silicon phase and refining θ phase. Furthermore, the effect of Ca was principally modifying primary silicon phase and primary α phase. The optimal microstructure was obtained when combined addition of 0.05%Sr-Ca was applied and the atomic ratio of Sr and Ca was 5 to 1, in which primary silicon phase and eutectic silicon phase as well as primary α phase were all modified significantly compared with those with only 0.05% Sr addition.