Valence band (VB) effective masses of nonpolar and semipolar nitride heterostructures were studied using k.p perturbation theory, with a particular focus on band mixing. When the absolute value of the energy separation of the topmost two VBs (vertical bar Delta E vertical bar) is sufficiently larger than the spin-orbit interaction (Delta(so)), the mixing between these two VBs was negligible. This enabled us to calculate an analytical expression for VB mass expression. Under this condition, the effective mass of the top VB was reduced less than 10% along a certain direction, and less than 25% for an in-plane (x'y'-plane) average, compared to the c-plane case. When Delta E was comparable with A(so), however, the VB structure became isotropic due to band mixing. Consequently, the VB band structure approached that of the c-plane and optical polarization was suppressed. On the other hand, VB mass along the growth direction (z') was always heavy, similar to that of the c-plane. However, the topmost two VBs had slightly different mass values along z', which affected radiative properties of non-c-plane nitride quantum wells (less than 2 nm wide) through the difference in quantum confinement strengths. (C) 2010 American Institute of Physics. [doi:10.1063/1.3448578]