On the basis of F S thin theory and taking the scattering of lattice vibration and impurities into account, the conductivity and the magnetoresistance of p type semiconducting diamond films (strip sample) are derived in spherical energy band and parallel connection model for light hold band,heavy hole band and split off band of diamond by solving the Boltzmann equation in the relaxation time approximation,. The relations between the magnetoresistance and thickness of diamond films,magnetic field strength and mobility are given. The magnetoresistance is influenced differently by light hole band,heavy-hole band and split off band. The deviation of the magnetoresistance is small between thick diamond films and lump. Magnetoresistive effects relate closely with temperature,magnetic filed strength and mobility.