Freestanding ultrananocrystalline diamond (UNCD) films with homojunction insulating layer in situ grown on a conducting layer showed superior electron field emission (EFE) properties. The insulating layer of the films contains large dendrite type grains (400–600 nm in size), whereas the conducting layer contains nanosize equi-axed grains (5–20 nm in size) separated by grain boundaries of about 0.5–1 nm in width. The conducting layer possesses n-type (or semimetallic) conductivity of about 5.6 × 10–3 (Ω cm)−1, with sheet carrier concentration of about 1.4 × 1012 cm–2, which is ascribed to in situ doping of Li-species from LiNbO3 substrates during growth of the films. The conducting layer intimately contacts the bottom electrodes (Cu-foil) by without forming the Schottky barrier, form homojunction with the insulating layer that facilitates injection of electrons into conduction band of diamond, and readily field emitted at low applied field. The EFE of freestanding UNCD films could be turned on at a low field of E0 = 10.0 V/μm, attaining EFE current density of 0.2 mA/cm2 at an applied field of 18.0 V/μm, which is superior to the EFE properties of UNCD films grown on Si substrates with the same chemical vapor deposition (CVD) process. Such an observation reveals the importance in the formation of homojunction on enhancing the EFE properties of materials. The large grain granular structure of the freestanding UNCD films is more robust against harsh environment and shows high potential toward diamond based electronic applications.