The present work shows that the field electron emission (FEE) properties of polycrystalline diamond films can be enhanced by control over the film thickness. The FEE properties of undoped, continuous, and smooth submicron-thick diamond films with initial nucleation densities of ∼5×1010 particles/cm2 were investigated as a function of diamond film thickness. A set of films with thickness ranging from 70–100 to 830 nm yielded turn-on field values of 6–8 V/μm and threshold field values of 8.5–17.5 V/μm (for 0.3 μA/cm2), respectively, without any conditioning. It was found that the films of thickness up to ∼370 nm can sustain stable current density as high as 0.1 A/cm2 without morphological modification. The thicker films, however, suffer from a strong degradation of the film and breakdown. The best FEE (lower turn-on and threshold fields and morphological stability) was obtained for a thin (100 nm) continuous diamond film. This result is suggested to be attributed mainly to the efficient electron conduction from the back contact to the surface.