The stability of a high-beta tokamak plasma having circular crosssection against the high-n ballooning modes is reconsidered. In the high-beta case, the poloidal magnetic field yields a rather strong driving term to the ballooning modes, leading to a substantial distortion of the structure of the second stability region. The various effects of the magnetic shear, pressure gradient and the poloidal magnetic field parameter on the eigenfunctions and the eigenvalues of these modes are calculated. The results obtained give a comprehensive indication in predicting the ideal MHD ballooning modes in circular tokamaks.