We report hole effective mass calculations ofSi1−xCxandSi1−yGeyalloys. All calculations are based on a 16×16 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin-orbit interaction taken into consideration. The 1 meV constant energy surfaces below the valence band edge are used to determine the nominal hole effective masses. The effective masses of light hole and heavy hole ofSi1−yGeyalloys vary as linear functions of Ge content and increase linearly as the hole energy increases from 1 to 15 meV. The heavy hole effective masses ofSi1−xCxalloys, however, exhibit a totally different trend. The effective mass ofSi1−xCxremains relatively unchanged fromx=0.0tox=0.9, and increases abruptly by a factor of two fromx=0.9tox=1.0. The nonparabolicity increases as the C content rises up tox=0.9, and nearly disappears when turning into pure diamond. The interaction between the split-off hole band and the heavy hole band is proposed for the anomalous behavior of the heavy hole effective masses of SiC alloys. ©1997 American Institute of Physics.