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. Based on the Fuchs and Sondheimer thin film theory (FS 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 calculation 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 .