Diamond films with various thicknesses (0.15-9 mum) were grown by microwave plasma chemical vapor deposition. The lowest threshold field strength for electron emission was 3 V/mum for the similar to1.5-mum-thick diamond film. The results were analyzed by effective emission areas and effective work function according to Fowler-Nordheim theory. It was found that the threshold voltage was strongly affected by the ratio of (111) and (110) oriented grains in the films. The larger the fraction of (111) oriented grains, the lower the effective work function in agreement with the reported negative electron affinity of (111) surfaces.