The crystallization kinetics of quasicrystalline decagonal phase in a rapidly quenched Al80Mn20 alloy has been investigated. The results of DSC experiments show that the activation energy for crystallization is a constant during the whole process. The experimental curve showing kinetic mechanism of crystallization has been obtained. The activation energy and frequency factor are determined to be 224 kJ/nol and 1.2×1012/s, respectively, The analysts of the kinetics indecates that the crystallization of decagonal phase is controlled by long range atomic diffusion, and the nucleation rate should be zero during the process.