Tribological properties of ultra nanocrystalline diamond (UNCD) films have chemically been modified by N+ ion implantation and subsequent annealing processes. Friction coefficient is found to be 0.15 in as-prepared film comparing to 0.09 and 0.05 in N+ ion implanted and post-annealed films, respectively. Such a modification of friction coefficient is a characteristic of the transformation of sp3 to graphitized/amorphized sp2 bonded carbon network. Transformation of sp3 to sp2 carbon network causes conversion of higher surface energy state (hydrophilic) to lower (hydrophobic) one which results in ultra low friction coefficient. Graphitization/amorphization in wear track observed by micro Raman spectroscopy is found to be the prominent mechanism for the reduction in friction coefficient.