Abstract: The stability of leeward streamwise vortices over a Mach 6 yawed cone with 6 degree angle of attack is investigated by using direct numerical simulation (DNS) and stability analyses including spatial BiGlobal and plane-marching parabolized stability equations (PSE3D). It is found that a pair of strong streamwise vortices inducing low-speed mushroom structure simultaneously emerge in the vicinity of the leeward plane. Theoretical results indicate that both low-frequency sinuous modes and high-frequency varicose modes may play an important role in the breakdown of the streamwise vortices.