The effect of water and acrylonitrile (AN) on apparent melting temperature of acrylonitrile/methylacrylate copolymer (PAN) wa investigated. Apparent melting temperature of PAN decresed with increasing water content and saturated at 150℃, at which water content was 25 wt％ to PAN. The water content corresponded to the molar ratio between water and the CN group in PAN was 1:1. When a mixture of water and AN (70/30 by weight) was added to PAN, apparent melting temperature continously decreased. Monophase PAN melt was obtained at a molar ratio of the CN group in PAN/AN/water = 2/1/2.