Mechanical properties and thermal conductivity of composites made of nanodiamond with epoxy polymer binder have been studied in a wide range of nanodiamond concentrations (0-25. vol.%). In contrast to composites with a low content of nanodiamond, where only small to moderate improvements in mechanical properties were reported before, the composites with 25. vol.% nanodiamond showed an unprecedented increase in Young's modulus (up to 470%) and hardness (up to 300%) as compared to neat epoxy. A significant increase in scratch resistance and thermal conductivity of the composites were observed as well. The improved thermal conductivity of the composites with high contents of nanodiamond is explained by direct contacts between single diamond nanoparticles forming an interconnected network held together by a polymer binder.