Abstract: The colour grade of the colourless synthetic diamond produced recently by the American Chatham Company is close to F. The highest clarity reaches Grade SI₁. The crystal is in the combination form mainly consisting of such simple forms as 111,100 and 211. The 110 simple form often does not grow perfectly. The lamellas growth veins is diagnostic on the corresponding crystal face. The absorption spectrum shows that nearly no impure nitrogen atoms are present in the crystal lattice of this kind of synthetic diamond. However, a few boron atoms (2802 cm^-1) still present in the crystal lattice. Therefore, this variety of synthetic diamond belongs to a mixed variety of Ⅱa～Ⅱb types. At the room temperature, the characteristic absorption peak only occurs at 271 nm in the ultraviolet with the ultraviolet absorption limited within 235 nm. Stimulated by cathode ray, this synthetic diamond produces strong blue-white fluorescence and displays its specific cathodoluminescent geometric patterns, growth subdivisions and mosaic structure. The blue cathodoluminescent peak is located at 432.5 nm, with some individual sample associated with weak luminescence at 575 nm. The alloy inclusions associated with the composition of the contact agent are in different shapes. The EPMA-EDS energy spectral analysis shows that the chemical composition of the alloy inclusions consists primarily of Fe (65 %~72 %), and secondarily of Cu(10 %~12 %), and also a litlle of such elements as S and Si. Therefore, an approach to the identification of Chatham synthetic colourless diamond is presented under the normal testing condition.