Synthesis of microcrystalline-ultrananocrystalline composite diamond (MCD-UNCD) films, which exhibit marvelous electron field emission (EFE) properties, was reported. The EFE of MCD-UNCD composite diamond film can be turned on at a low field as 6.5 V/μm and attain large EFE current density about 1.0 mA/cm2 at 30 V/μm applied field, which is better than the EFE behavior of the nondoped planar diamond films ever reported. The MCD-UNCD films were grown by a two-step microwave plasma enhanced chemical vapor deposition (MPECVD) process, including forming an UNCD layer in CH4/Ar plasma that contains no extra H2, followed by growing MCD layer using CH4/H2/Ar plasma that contains large proportion of H2. Microstructure examinations using high resolution transmission electron microscopy shows that the secondary MPECVD process modifies the granular structure of the UNCD layer, instead of forming a large grain diamond layer on top of UNCD films. The MCD-UNCD composite diamond films consist of numerous ultrasmall grains (∼5 nm in size), surrounding large grains about hundreds of nanometer in size. Moreover, there exist abundant nanographites in the interfacial region between the grains that were presumed to form interconnected channels for electron transport, resulting in superior EFE properties for MCD-UNCD composite films.