A phosphorus (P)-containing diamine was synthesized and used as a comonomer at a minor fraction to improve insufficient non-flammability of a poly(ester imide) system as a promising candidate of base film materials in flexible printed circuit boards (FPC). The P-modified PEsI copolymer film displayed the highest level of non-flammability (UL-94, V-0) comparable to commercially available conventional aromatic PI films in addition to original excellent properties, i.e., a very high T_g, a low linear coefficient of thermal expansion (15.7 ppm/K) close to copper foils, a low water absorption (0.54 %), a low linear coefficient of humidity expansion (5.53 ppm/RH%), and sufficient toughness (elongation at break = ca. 40%). This approach was also attempted for a very flammable low-modulus PI system for aiming at an application as a coating-type cover layer material in FPC.