To understand the reason of creep rupture life of superalloy K424 drops sometimes under investment casting process, the defects in the test bars were investigated on the shape, distribution with the help of scanning electron microscope (SEM) and microhardness tester. The causes of the defects and influences on creep rupture were analysed. The results show that there are micro shrinkage porosities in interdendritic regions and intact regular octahedron shaped MC carbides can be founded nearby. The microsegregation of the elements is in different degrees which the most serious element is Cr. The cause of the micro shrinkage porosities is that the adjacent dendrite arms join each other and split liquid alloy into closed parts. The micro shrinkage porosities cause stress concentration in the test bars and thus cause the lower high temperature creep rupture life.