The effect of a small amount of nitrogen incorporated in chemical vapor deposited diamond films on their electrical properties was studied. Upgrading the purity ofCH4gas from the conventional ultra-high purity to the grade with no detection of nitrogen resulted in improvement of electrical properties of the boron-doped homoepitaxial diamond films; the Hall mobility was increased by 4.3 times at the room temperature. Decrease in nitrogen concentration in the diamond films was confirmed by investigating the 2.16 eV center of cathodoluminescence induced by ion beam irradiation and subsequent annealing. ©1997 American Institute of Physics.