Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H2/CH4/N2 gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5-6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C2 species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H2/CH4 gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H2/CH4/N2 plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer Hα line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/Hα ratios with the coating roughness and grain size were also confirmed with different set of gas flows/ plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H2/CH4/N2 gas mixture. The RMS roughness of 4 nm and grain size of 5-6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/Hα species in the plasma. Wear tests were performed on the OrthoPOD®, a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.