The mechanism of electron field emission from CVD diamond films is investigated while analyzing its structure. The results indicate that there is some graphite in CVD diamond films and the coefficient which electrons tunneled through the diamond grains is enhanced because the electrons obtained energy from the electric field in graphite. Based on the results, a mechanism is put forward that enhancement of the electrons tunneled through diamond grains by graphite resulted in improvement electron emission from CVD diamond films. Some experimental results are well explained by the mechanism.