Based on the Fuchs and Sondheimer thin film theory (F-S film theory) and a revised valence band split-off model, considering a mixed scattering by lattice vibrations, ionized impurities and surfaces, a theoretical description of the piezoresistive effect (PR effect) in p-type heteroepitaxial diamond films was presented by solving the Boltzmann transport equation in the relaxation time approximation and using the parallel connection resistance model. A calculating expression of the PR effect was given. The main characteristics that were identical with the experiment were obtained by theoretical calculation. Giving out a model to show that the energy level interval between the split-off band and the heavy-hole band was changed by strain, a reasonable explanation was presented for the error between experimental results and theoretical values of saturated PR effect under big strain.