Microwave assisted switching of magnetization has become one of the key techniques for future advanced recording. We have studied the magnetization switching behaviors excited by circularly and linearly polarized microwaves based on the Landau-Lifshitz-Gilbert equation. The precessional motion of magnetization is excited by the microwave, resulting in significant reduction of the irreversible reversal field. The reversal takes place through very complicated precessional motion, which proceeds via nearly steady precession at an early stage, subsequent unstable motion, and finally abrupt irreversible switching. We discuss the detailed effect of microwave polarization on magnetization reversals.