The mechanical properties of DD3 superalloy solidified at various undercoolings were measured to investigate the effects of gamma' precipitate and microstructure on alloy properties. The influence of melt undercooling on the gamma' precipitation is also studied. It is found that not only the size of gamma' particle, but its distribution in the as-solidified structure is also drastically controlled by the melt undercooling. The analysis indicates that alloy solidified at a low undercooling is brittle, thus leading to a lower toughness and tensile strength. With increasing undercooling, the toughness and strength of the alloy increased accordingly, which may be attributed to the strengthening effect of gamma' precipitate and the reduced micro-segregation.