Abstract: Physical mechanisms of hypersonic boundary layer transition induced by diamond roughness element are investigated by means of direct numerical simulation (DNS) from the aspects of topological structure stability and hydrodynamic stability. Topological structure stability theory reveals the existence of unstable connection orbit between saddle-saddle points in front of the roughness element and unstable connection orbit between saddle-node-saddle points behind it. Consequently, unsteady and asymmetric structures are formed owing to the influence of disturbance. The hydrodynamic stability analysis shows that high-frequency disturbing waves are invoked by the diamond roughness element and large-scale vortices tend to break up along the development of disturbance. The effects of both instabilities coexist in the flow field. In addition, comparisons are made between different roughness types (cylindrical, ramp, diamond), analyzed transition mechanism of different roughness types, support design of hypersonic forced-transition device in the theory.