The effect of the N2 and H2 addition in Ar plasma on the characteristics of the UNCD films was systematically investigated. It is found that, while the N2/Ar plasma results in UNCD films with ultra-small grains (~ 5 nm), incorporation of H2 into the N2/Ar plasma increased monotonously the size of the grains. Moreover, the diamond grains synthesized in H2 free plasma are of equi-axed geometry and those grown in H2-containing plasma are of plate-like one. The optical emission spectroscopic investigation indicated that the increase in electron temperature due to the addition of H2 into Ar plasma is the main cause, altering the microstructure of the UNCD films. As the H2 content increases, the spherical diamond grains first agglomerated to form elongated grains, which coalesce to form dendrite clusters. The proportion of grain boundaries is thus decreased that increased the turn-on field necessary for inducing the electron field emission process.