Using EPMA and HRTEM, a systematic study is conducted on the gold-bearing arsenopyrites from Huangjindong gold deposit, Hunan province, by means of BSE observation, X-ray scanning, quantitative analyses and line analyses and HRTEM determination of crystal structure. It is revealed that no nanometer-grained or less grade inclusion gold grains are found, and that the structure of the arsenopyrites develops perfectly without any "abnormally structural part" in them. This shows that Au elements are scattered in the whole ar-senopyrite crystal, i. e. , the gold exists in structural state or chemical state. It is also found that Au content varies substantially in different crystal striations, therefore it is proposed that Au came into the crystal structure of the arsenopyrites when they formed. The entrance of Au into the crystal structure results in widening of the distance of the crystal face (100) (d100) and lightly variation of the unit cell parameters, the crystallographic axis a becoming longer. Based on the detailed analysis, it is believed that Au mainly got into the the arsenopyrites in form of Au3+ to have replaced Fe3+ in the AsS3-octahedron spaces and that the variation of Eh values is possibly a most important factor to control the formation of the structural gold.