In order to comprehensively understand the forming mechanism of abnormal phases solidified in a nickel-base cast superalloy with additives of tungsten and molybdenum, the coupling effects of W and Mo on the micro-structure and stress-rupture properties were investigated in this paper. The results indicated that the pre-cipitation of primary α-(W,Mo)phase depended tremendously on the amount of W and Mo addition.When the total amount of W and Mo was greater than 5.79 at%,α-(W,Mo)phase became easily precipitated in the alloy. With increasing of Mo/W ratio,the dendrite-like α-(W,Mo)phases were apt to convert into small bars or blocky-like phases at the vicinities of γ′/γ eutectic.The morphological changes of α-(W,Mo)phase can be interpreted as the non-equilibrium solidification of W and Mo in the alloy. Since the large sized α-(W, Mo) phase has detri-mental effects on stress-rupture properties in as-cast conditions, secondary cracks may mainly initiate at and then propagate along the interfaces of brittle phases and soft matrix.During exposing at 1100℃for 1000 h,the α-(W, Mo) phases transformed gradually into bigger and harder M6C carbide, which results in decreasing of stress-rupture properties of the alloy. Finally, the alloy with an addition of 14W-1Mo(wt%) maintained the longest stress lives at high temperatures and therefore it revealed the best microstructure stability after 1100℃/1000 h thermal exposure.