We have studied the field emission characteristics of defective diamond films grown by microwave plasma enhanced chemical vapor deposition. X-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and the reflectance measurements have revealed the poor crystal quality and/or small grain sizes of the diamond phase and the inclusion of the non-diamond carbon phases in these films. The degrees of the film defectiveness have been found to depend on the methane concentration. Current-versus-voltage measurements have demonstrated that the defective diamond films have good electron emission characteristics, and, moreover, that the emission characteristics depend on the methane concentration. The observed correlation between the degrees of film defectiveness and the emission characteristics strongly suggests the defect-related electron-emission mechanism. In this study, the lower limits of the field emission current densities, estimated by averaging the total measured emission currents over the entire sample areas, of 1 mu A/cm(2) and 1 mA/cm(2) have been measured at electric fields as low as 4.5 and 7.6 V/mu m, respectively. We have also examined the reproducibility, the uniformity, and the stability of emission currents.