Tribological properties of plasma chemistry dependent ultrananocrystalline diamond (UNCD) films deposited by microwave plasma enhanced chemical vapor deposition system are studied using 100Cr6 steel ball as a sliding counter element. UNCD films synthesized with 1.5% H2 in Ar/CH4 plasma consist of large number of clustered diamond grains with small volume fraction of grain boundaries. Decrease in grain boundary volume fraction resulted in simultaneous decline in trans-polyacetylene (t-PA) chain and sp2 bonded amorphous carbon (a-C) phase fraction. However, UNCD films grown in Ar/CH4 plasma shows formation of UNCD grains with high volume fraction of grain boundaries, large amount of sp2 bonding, a-C and network of t-PA chains. The t-PA passivates the dangling bonds and sp2/a-C phase that externally reduces sliding shear resistance, thereby, resulting in ultra-low friction behavior of UNCD films deposited from Ar/CH4 plasma medium. Load dependent frictional behavior of these films was investigated in present study. These films tested under both low and high normal loadings showed high friction coefficient. Capillary dominated surface interaction and plastic deformation based models were found to be appropriate for describing friction behavior under low and high loads, respectively. However, at specific and intermediate normal loads, both low and ultra-low friction coefficients could be explained through elastic contact model that is accounted by the presence of lubricious phase of sp2 and a-C. In addition, low friction coefficient is also governed by surface passivation mechanism. This mechanism is explained when friction test is carried out in controlled atmospheric condition. Investigating the tribological behavior of these films against steel ball is useful for implementing reliable micro-sliding based device applications.