Abstract: Icosahedral diamond crystals (IDCs) were found in chemical vapour deposited diamond by a field emission scanning electron microscope, using. YG13 (cemented tungsten carbide containing 13% of Co) as a substrate. It is found that the nanometer sized IDCs are in the shape of a nearly perfect icosahedron and the micrometer sized IDCs are in the shape of a icosahedron with dimples or grooves. A simulation on atomistic scale is performed by assuming a nucleation-growth model in order to elaborate the mechanism of IDC formation. Results show that the nucleus of IDCs is dodecahedrane, C₂₀H₂₀. For the tetrahedral cubic packing, the dihedral angle of 111 facets of IDCs is 2.87^o larger than that of the normal 111 twin plane. The geometric position of the bilateral atoms on the 111 twin plane of IDCs is mismatched, resulting in a distorted twin plane (DTP) in IDCs. The high energy of DTP induces the dimples or grooves that occurred in IDCs.