In order to investigate the differences of the transition mechanisms between four roughness shapes, a fifth-order WCNS scheme was used to simulate the supersonic flat-plate boundary layer transition induced by the square, cylinder, diamond and hemisphere roughness elements at Mach 4.20. Results show that the square roughness element has the longest separated region where the absolute instability is formed, thus resulting in the earliest transition. The diamond roughness element has the widest separated region and leads to the widest turbulent wake region. The cylinder and hemisphere roughness elements are less effective in inducing transition compared with the square and diamond roughness elements.