Abstract: The magnetoresistive effects were investigated at different temperature and magnetic filed B in both homoepitaxial and heteroepitaxial p-type semiconducting diamond thin films. There are two kinds of device structure used i.e. strip and Corbino disk. Experimental results show that the changes of resistance in the film strongly depend on the magnetic field B, temperature and the geometric form of samples. The magnetoresistance of disk structure is greater than that of strip sample, and the effects of strip sample increase with the ratio of width-to-length. Based on the Fuchs and Sondheimer thin film theory (F-S theory), the magnetoresistances of strip and disk are calculated at magnetic field of 5 T. It is 0.38 for strip and 0.74 for disk. The influence of Hall effect on the magnetoresistance is discussed and a possible mechanism of geometric form effect is described.