Abstract: Using an eighth-order set of determinate, forced, dissipative, and nonlinear autonomous equations, which governs the dynamics of a quasi-geostrophic flow of the baroclinic atmosphere, the chaotic forms of large-scale atmospheric motion are investigated numerically. In certain ranges of the thermal forced parameters, the chaotic forms of the variations of the flow patterns are exhibited distinctly. Along with gradual change of the thermal forced parameter, trans-formation; between periodic and chaotic motion forms occur repeatedly.The windows of period-3 and period-6 emerge also in the chaotic regions. The periodic motion forms of the flow patterns corresponde to the periodic low-frequency vacillations of the subtropical high cells along the zonal direction during summer half year. After the transformation from periodic to chaotic form occurs, these regular vacillations are broken immediately. This phenomenon of the establishing and the breaking of the periodic low-frequency vacillation of the subtropical high cells along the zonal direction tallies with observational fact.