By introducing positive substrate bias, ranging from 0 to 200 V, to the substrate during the growing procedure of diamond at 1, 2, and 3% CH4 concentration, we have investigated the effect of electron bombardment on modification of the structural property and the surface morphology of diamond films, and consequently the field emission properties. When increasing the bias voltage for each CH4 concentration, the structural properties of diamond films are significantly deteriorated together while the non-diamond carbon component increased and the surface morphologies of the films lost their unique facet shape. The reason for the deterioration of the structural property was attributed to both the increase of substrate temperature and the excessive generation of CHn radicals. Especially for the films deposited at 2% CH4 concentration under 100 V, it was observed that their morphological and structural characteristics approached those of graphitic carbon nature. The field emission properties of diamond films were substantially improved with increasing the CH4 concentration and with the application of bias voltage for each CH4 concentration. In order to investigate the correlation between the enhancement of field emission properties and the emission sites, we have examined the spatial distribution of the emission sites. From this result, a possible emission mechanism is discussed.