An extended tight-binding thermochemical model is proposed to determine the band offsets of group III-nitride heterostructures as a function of temperature, pressure, and strain. The Valence and conduction band energies of semiconductors at 0 K are first obtained using light-binding model, with the Hartree-Fock atomic term values and considering the nonorthogonality of p-orbitals of adjacent atoms and atomic spin-orbit splittings of p-orbitals. Temperature, strain, and pressure effects on conduction and valence band energies are then determined using the thermochemical model which considers the free electrons (e(-)) and free holes (e(+)) as weakly interacting quasichemical particles in intrinsic semiconductors. Finally, the valence band offsets are determined by taking the difference between the valence band energies of constituent semiconductors which are screened by optical dielectric constants at any temperature, pressure, and strain. Excellent agree ment is found between the theory and experiment.