Abstract: The structure of AlN HTCC microwave substrate structure possesses was introduced and the microwave properties and reliabilities were studied in this paper. The results show that the microwave transition insert loss decreased slowly at the wave frequencies from 5 GHz to 25GHz and rapidly at the frequency of 25 GHz. And it can be used at microwave frequencies lower than 25 GHz. And the properties of AlN HTCC substrate can meet the SMT processes such as Au wire bonding and SnPb soldering after high temperature storage, thermal shock and damp heat test can meet the SMT processes. After damp heat test, a little bit change such as water stains can be observed on  Au plating layer and corrosion marks on AlN ceramics. And remarkable corrosion can be observed on the Au plating layer after Salt spray test. So the AlN HTCC substrate should not be used in humidity and salt spray environment. And necessary protections should be required.