In this study, thermal conductive composites with various filling fraction were prepared using angular and spherical AlN powders as fillers and PDMS as organic matrix. The influence of particle morphology, filling fraction and surface treatment on the viscosity and thermal conductivity of composites were deeply investigated. The results indicated that the AlN spherical powders could significantly reduce the viscosity of composites compared with the AlN angular powders, which was in favor of obtaining higher filling fraction and larger composite thermal conductivity. In addition, the surface treatment of AlN powders with KH570 were beneficial to reduce the interface thermal resistance and improve the composite thermal conductivity. When the concentration of KH570 was 2.0wt%, a highest composite thermal conductivity were obtained, which was ~22.5% higher than that of untreated fillers.