Stretching induces to form electric interaction between backbone and mesogenic side chain in the bulk of a novel polyimide fiber that was synthesized from 4,4'-oxydiphthalicanhydride (ODPA) and 6-(4-phenyl phenoxy) hexyl-3,5-diaminobenzoate (DABBE6), which results in unusual in situ self-reinforced function, and surprisingly, zigzag undulation of mechanical properties of the annealed polyimide fibers with increasing draw ratios. X-ray scattering and differential scanning calorimetry (DSC) results indicate that some order structures develop in the annealed fibers under tensile stress. Dynamic thermomechanical analysis (DMA) and DSC results also indicate that no phase separation is observed between backbone and side chain. Further results of experiments and computer molecular simulation prove that the biphenyl side chain can be parallel to the backbone via methylene spacers bending and together align orientationally under tensile stress to develop electronic interaction, and the molecular conformations are stabilized to form binding-like molecular structures by the interaction to enhance strength and rigidity of backbone. Meanwhile, formation of the interaction depends on the relative position between backbone and the trail phenyl group of side chain, which contributes to the zigzag undulation of tensile strength and initial modulus of the polyimide fibers. In addition, relationship of draw ratios and formation of the interaction was established, and a corresponding backbone-side chain-packing model was suggested. POLYM. ENG. SCI., 49:1225-1233, 2009. (C) 2009 Society of Plastics Engineers