Field emission characteristics for the diamond films grown using a gas mixture of different methane concentration in hydrogen were investigated. Measured by using the field emitter with diode structure, the turn-on voltage (field) for emitting 0.1 mA/cm2 current density and the critical electric field for 10 mA/cm2 were 5 V (3.0 V/μm) and 9 V (5.5 V/μm), respectively, for the diamond emitter of a little poor quality grown in 1.5% methane concentration. While, for the good quality diamond emitter grown in 0.5% methane concentration, the values were shown as 10 V (6.1 V/μm) and 21 V (12.7 V/μm), respectively. It is suggested that this phenomenon can be related with the field enhancement effect due to protruding small crystallites and the energy bands induced by internal stress or defects, depending on the film quality. © 1998 American Vacuum Society.