Polyamic acid (PAA) based on pyromellitic dianhydride and 4,4'-oxydianiline (PMDA-ODA) can be chemically converted to copolyamic acid-imide (PA-I) which is a partially converted PAA at ambient temperature by treatment with a mixture of acetic anhydride and pyridine. A systematic investigation by rotation viscometer and FT-IR revealed that phase separation in PA-I solution occurred when imidization degree was about 26%. Imidization degree of PA-I solution could be controlled by using adequate acetic anhydride amount, initial PAA concentration and imidization time. PA-I solutions with different imidization degree were converted to corresponding polyimide films by thermal imidization. WAXD showed that as imidization degree in PA-I solution increased, the degree of morphologic ordering of final PI film was improved, while tensile strength and modulus increased and ultimate elongation reduced slightly.