Owing to its superior mechanical properties, nanodiamond (ND) holds great potential to improve tribological characteristics of composites. In this study, we report on the wear and dry friction of epoxy-ND composites prepared from as-received and aminated ND across the length scale range from macro to nano. Comparison of macroscale, microscale, and nanoscale frictional behavior shows that ND is highly effective in improving the wear resistance and friction coefficients of polymer matrices across the different length scales. Although with both types of ND wear resistance and friction coefficients of epoxy-ND composites were significantly improved, aminated ND outperformed as-received ND, which we account to the formation of a strong interface between aminated ND and the epoxy matrix. This study also shows that agglomerates within epoxy-ND composites containing 25 vol.% ND were able to wear an alumina counterbody, indicating very high hardness and Young's modulus of these agglomerates, that can eventually replace micron sized diamonds currently used in industrial abrasive applications.