Velocity fields and temperature fields are numerically analysed using an unsteady mathematical model for mixed convection in a rectangular cavity with symmetric and unsymmetrical top supplying air.The Rayleigh number Ra is fixed at 10~6 and Reynolds number Re is given in a range of(1 000) to 3 000.The numerical results obtained show that increasing the Reynolds number the numerical solutions change from steady symmetric state to periodically oscillatory and then to non-periodically oscillatory one for the problem with symmetric top supplying air.However,numerical solutions obtained are steady for the problem with unsymmetrical top supplying air though the maximum grid Peclet number Pe is greater than that with symmetric top supplying air.Therefore,it can be illustrated that the solution oscillation is a physical oscillation for the mixed convection in a rectangular cavity with symmetric top supplying air,and it is not a numerical oscillation.