The kinetics of propylene polymerization using homogeneous Cp2ZrCl2/methylalumoxane (MAO) catalysts have been investigated at 40 degrees C and 1 atm of propylene pressure. The effects of [Al]/[Zr] and [Zr] on the catalyst activity were studied. It was found that MAO plays a critical role in catalyst activation and deactivation. By assuming the coexistence of two dynamic equilibria between active species and inactive ones which are the products of zirconocene dimerization and zirconocene-MAO complexation, a kinetic model was proposed. The model could satisfactorily explain most of the experimental results.