The effect of electron bombardment on modification of the structural property and the surface morphology of diamond films, and consequently the field emission properties have been investigated by introducing positive substrate bias. When increasing the bias voltage, the structural properties of diamond films are significantly deteriorated together with the increase of nondiamond carbon component and the surface morphologies of the films lost their unique facet shape. The reason for the increase of nondiamond carbon content is described in terms of both the increase of substrate temperature and the excessive generation of CHn radicals. The field emission properties of diamond films were substantially improved with increasing bias voltage. Enhancement of field emission property for the firms prepared by increasing bias voltage and/or methane concentration has been discussed by correlating between the substrate-diamond interface property and the slope of the Fowler-Nordheim plot.