Site occupancies of ternary additions (Ti, V, and W) in the C15 ZrCr2 and NbCr2 Laves phases were predicted theoretically by first-principles calculations based on density functional theory. The results suggest that Ti preferentially occupies the Zr and Nb sites in ZrCr2 and NbCr2, respectively. V and W substitute the Cr sites in both ZrCr2 and NbCr2. The calculations of heats of formation also show that the occupancy of W on the Cr sites and of Ti on the Zr sites stabilize ZrCr2. For NbCr2, the occupancy of V on the Cr sites and of Ti on the Nb sites increases the phase stability.