Abstract: In recent years, China has become the largest producer of type Ⅱa colourless HPHT synthetic diamonds. And the identification of type Ⅱa HPHT synthetic diamond has become a challenging task. This research attempted to find the evidence of identification for type Ⅱa HPHT synthetic diamonds. Diamond samples from the high pressure and high temperature (HPHT) manufacturers in China (HH, J, Z) were collected, from which the inclusions and the distribution of impurities of 11 typical type Ⅱa diamond samples produced in the past three years were studied. Two main points of understanding were obtained: (1) When the fluorescent colour and image of type Ⅱa diamond under Diamondview^TM and other equipments can not give clear identification evidence, the conical, black thread-like or thin tubular inclusions, the distribution of fine point inclusions, shapeless cohenite, hematite, and magnetite inclusions can be used as the reference for determining the new HPHT synthetic diamonds. (2) Some samples have impurity defects which can be tested, and their emission peaks and distributions which are related to Ni and B can be used as the contributory characteristics for determining HPHT synthetic diamonds.