Load dependent anomalous behavior of friction coefficient in ultra-nanocrystalline diamond (UNCD) film was investigated while sliding against SiC ball. At low and high loads, the friction coefficient was quite high. However, at intermediate loads, ultra- low friction coefficient 0.002 was measured. Raman analysis of sliding interfaces showed evolution of high fraction of a-C/sp2C=C bonding at low and high loads. This invariably results in occurrences of unsaturated carbon atoms which chemically forms strong bonding. However, wear rate does not follow the trend of friction coefficient and found to increase with load. Such a behavior indicates that friction coefficient depends on the nature of chemical bonding present at sliding interfaces.