A series of novel co-polynaphthalimides (IIIa-h) were synthesized from 1,4,5,8-naphthalene tetracarboxylic dianhydride with 9,9-bis(4-aminophenyl)fluorine and its equimolar mixtures with various aromatic diamines by high-temperature solution ploycondensation. Series III had inherent viscosity ranging from 0.77-1.17 dL/g, respectively. Most of the films were soluble in m-cresol and some of them were dissolve in 1-methyl-2-pyrrolidone (NMP). These polymers could afford transparent and tough films with Strength at Break of 77-100 MPa via casting relative m-cresol solutions. These PNIs also exhibited high thermal stability, with glass-transition temperatures of 218–239 oC, 10% weight loss temperatures above 500 oC in nitrogen or air, and char yields at 800 oC in nitrogen higher than 40%.