The effect of Ta concentration on the fundamental mechanical properties of W–Ta alloys has been studied from first principles study. The lattice constants, the cell volumes, and the formation energies of the W1−x Ta x (x = 0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.5, 0.5625, 0.625, 0.75) alloys were calculated. It is shown that Ta alloying in bcc W lattice is an infinite solid solution and the W0.5Ta0.5 have the lowest formation energy. With the optimized geometry and lattice, the elastic constants are calculated and then the elastic moduli and other mechanical parameters are derived. Results show that although the mechanical strength of the W–Ta alloys is lower than that of pure W metal, it is much higher than that of pure Ta metal. On the other hand, the B/G ratio and the Poisson's ratio of the W–Ta alloys is much higher than that of pure W, and even higher than that of pure Ta, indicating that Ta alloying can improve the ductility of bcc W substantially.