Abstract: Dynamic mechanical analysis (DMA) is an important method to study the cure process of prepregs. It was used to investigate the isothermal curing of the 648 epoxy prepreg. Based on the relative conversion definition of the increase in storage modulus E′，the evolution of E′ at different isothermal temperatures was studied. It was found that the increase of E′ during isothermal cure process at high temperatures is different from the one at low temperatures. The low temperature conversion curve includes a transitional stage which was not observed in the high temperature curves，and this transitional stage is found to be strongly frequency-dependent. Hsich‘s non-equilibrium thermodynamic fluctuation theory and Avrami equation are fitted to the conversion curves of E′，and can describe the evolution of E′ by two separate stages. The value and evolution of activation energy calculated by Avrami equation and the isoconversional methods are different in different temperature regions.