A series of high molecular weight PI precursors, poly(p-phenylene biphenyltetracarboxamide acid), were synthesized from 3,4,3',4'-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) by using intense mechanical stirring at -15 to 0 degrees C for 48-72 h. The as-synthesized PI precursor solution was used to make BPDA/PDA polyimide thin films and electrospun nanofibers. IR, Ostward Viscometer, CMT-8102 Electromechanical Universal Testing Machine and scanning electron microscope (SEM) were used for the characterizations of the as-synthesized PI precursor, PI films and nanofiber sheets. The high molecular weight BPDA/PDA PI thin films and electrospun nanofiber sheets possess excellent mechanical properties of up to 900 MPa tensile strength with up to 18.0 GPa E-modulus and up to 2 10 MPa tensile strength with up to 2.5 GPa E-modulus, respectively. (C) 2005 Elsevier Ltd. All rights reserved.