The effects of Mo addition and aging temperatures on phase constitution, microstructure and room/high-temperature mechanical behavior of the Co-9Al-xMo-(9–x)W-2Ta-0.02 B alloys (x = 4, 9, referred as 4Mo and 9Mo alloy hereafter) prepared by arc-melting were investigated. Results show that the as-cast alloys are composed of Co-base solid solution γ phase and intermetallic Co3(Al, Me) compound γ’-phase, where Me stands for W, Mo and Ta. The γ phase is the matrix and the γ’ phase distributes on the grain boundaries of the γ matrix. While after 1350 °C/8 h solution and 800 °C/100 h aging, the cubic γ’ phase with a size of 200~300 nm homogeneously precipitates on the γ matrix for the 4Mo alloys. As for the 9Mo alloy, however, the microstructure is significantly changed, and there are a large number of needle-like and cellular к-Co3Mo precipitates besides a few cubic γ’ precipitates. After 1350 °C/8 h solution and 900 °C/100 h aging, a few of the needle-like к-Co3Mo precipitates appear in the 4Mo alloy, while the needle-like к-Co3Mo precipitates in the 9Mo alloy are coarsened. The 4Mo alloy presents excellent tensile mechanical properties, better than the 9Mo alloy at room and high temperature, and both alloys exhibit yield stress anomalies at temperatures above 600 °C