Abstract: In this paper, a numerical simulation on hypersonic wakeflow over a sphere is carried out to study the wake instability and itsevolving process with Mach number 6 and Reynolds number 1.71\times 10^6(based on the radius of sphere). Firstly, a stable solution of the flow is obtained;one primary separated zone and one second separated zone are presented inthe base flow while the flow is axisymmetric. Secondly, weakly unsteady flowhas been developed slowly in the base flow to induce the obvious structuralinstability and obtain the periodic solution with the dimensionless periodvalue of 12.0. Thirdly, the evolving process of the vortical structure andits singularity characteristic are also analyzed. The results indicate thatinstable phenomenon will exist in the base flow of hypersonic flow over asphere and the present simulation method has the ability to study thisinstability problem.