Regarding to the difficulties in measuring microwave dielectric properties of diamond films due to their characteristics of low dielectric loss and small thickness, a split-cylinder resonator apparatus was established. By measuring dielectric performances of sapphire samples with different diameters, the capability of the split-cylinder resonator for measuring low dielectric loss materials was demonstrated, and the influence of sample's diameter on measurement results was studied. Then by using the split-cylinder resonator apparatus, dielectric properties of high quality diamond films prepared by two different methods, microwave plasma chemical vapor deposition (MPCVD) and DC arc plasma jet, were measured in the Ka band. Results show that the diamond film deposited by MPCVD method has a higher quality than that of the diamond film deposited by the DC arc plasma jet method, which is consistent with the results of their Raman and UV-visible absorption spectra. The results indicate that both the relative dielectric constant and the loss tangent of the sample deposited by MPCVD method are lower than those of the sample prepared by DC arc plasma jet method.