The microstructural evolution and room/high-temperature mechanical properties were investigated for a Co-9Al-9W-2Ta alloy with 4at%, 6at% and 9at% Nb additions to replace the same content of W (referred as to 4Nb, 6Nb and 9Nb alloy hereafter, 0Nb for the alloy without Nb addition, and the content of W+Nb=9 at% for all alloys) prepared by arc-melting. Results show that the as-cast alloys exhibit a microstructure composed of Co-base solid solution γ phase (γ-CoSS) and eutectic of γ+ intermetallic compound Co3Nb. While after 1200 °C, 8 h solution and 800 °C, 100 h aging, the cubic γ’ phase with a size of 200~300 nm precipitates homogeneously and coherently on the γ matrix for the 0Nb~6Nb alloys. Besides, cellular and/or needle-like Co3Nb phases are found in the 4Nb~9Nb alloy. The 6Nb alloy shows the highest yield strength in the temperature range of 600~700 °C, while the 4Nb alloy has the highest strength at 800 °C. Alloys exhibit yield stress anomalies at temperatures above 600 °C, and the temperature corresponding to the anomaly stress peak is about 700 °C