We investigate the onset of chaotic reversals of thermal convection cell in a two–dimensional square cavity using direct numerical simulation. To our knowledge, the chaotic reversal motion at the lowest Rayleigh number is related to the unicellular motion in this system. As Rayleigh number increases, the two antisymmetric attractors, which arise from the supercritical Hopf bifurcation, approach each other. However, because the basin boundaries of these attractors have multiple unstable directions, the behavior of the global changes seems to be complex for the small range of Rayleigh number around the onset of the chaotic reversal. On the other hand, period–doubling cascade from periodic reversal solutions leads to chaotic reversal as Rayleigh number decreases. At the conference we will report observation about the beginning of reversal and the quantities of the chaotic attractor.