Melting and isothermal crystallization behaviors of plasticized polyacrylonitrile (PAN) on adding water in a sealed state were studied. As water was added, the melting point of PAN was depressed sharply to reach a plateau region in which all the nitrile groups of the polymer were hydrated. The crystallization exotherms of PAN/water mixture showed complex profiles. On DSC analysis, it was found that the exotherm from the crystallization (dehydration) heat of the hydrated PAN was overlapped with the latent heats by the neat PAN and water which was not evolved during quenching. Thus, in order to investigate the dehydration kinetics of PAN/water mixture, it was required to separate the exothermic peak by the isothermal crystallization from the peaks by the evolution of latent heats of the each component, i.e., PAN and water. By subtraction of the latent heat contributed by neat PAN and water from the all isothermal exothermic heat of PAN/water mixture, the isothermal crystallization peak of PAN/water mixture was successfully obtained according to various crystallization temperature ($T_c$).