Diamond-like carbon (DLC) exhibits excellent wear and friction characteristics. Transmission electron microscopy (TEM) has been used to investigate the substructures of as-deposited DLC and DLC debris after wear testing. The as-deposited DLC was found to consist of a dense, three-dimensional network structure with a medium range order (< 3 nm) present. Diffraction pattern analysis showed that DLC was mainly amorphous. Two diffuse diffraction rings with d(111) = 0.21 nm and d(220) = 0.12 nm were observed, suggesting the presence of a short-range cubic diamond structure (sp(3)). Morphologically, the wear debris was found to be a discontinuous segregation of carbon particles ranging from nano- to micro-size. Diffraction pattern analysis showed that the debris consisted of graphite (sp(2)) and distorted DLC (sp(3)). A wear mechanism has been proposed based on the transformation of DLC to graphite. The transformation is related to the frictional energy and includes two stages : hydrogen release from the structure causing lattice relaxation and shear deformation of the DLC structure producing graphite.