The heteroepitaxial diamond films were grown on the p-type Si (100) substrate by microwave plasma chemical vapor deposition (CVD). The diamond films were patterned to strip types with the oxygen plasma etching technology. The magnetoresistance measurements have been carried out in a magnetic field ranging from 0 to 5 T, using the four-probe method. It was shown that the heteroepitaxial diamond films may produce fairly big magnetoresistance. Based on the Fuchs and Sondheimer thin film theory, mixed scattering by lattice vibration,ionized impurities and surfaces is considered. Taking the parallel connection resistance model, a theoretical description of the magnetoresistive effect in heteroepitaxial diamond films is presented by solving the Boltzmann transport equation in the relaxation time approximation. A relationship between the magnetoresistance and the film thickness, magnetic field, mobility and hole density is developed. The influence of valence deformation and surface scattering to the magnetoresistance of heteroepitaxial diamond films is discussed. A possible cause is proposed to explain the fairly big magnetoresistance of the films.