The degree of in-plane molecular orientation of various polyimides (PIs) and their precursors, poly(amic acid)s (PAAs), were estimated by measuring the visible dichroic absorption at an incidence angle for a rod-like dye (perylenediimide, PEDI) dispersed in the matrices. The effects of PI chain structure, film thickness, heating rate, and residual solvent on a spontaneous in-plane orientation phenomenon were examined to fully understand the mechanism. All PAA films cast on a substrate showed the low degrees of in-plane orientation of the chain axis, nearly independent of the chain structure. Upon thermal imidization of the PAA films adhered on a substrate, a striking spontaneous in-plane orientation behavior was observed for some PI systems with rigid chains; in contrast to that, some flexible PI systems showed no spontaneous behavior. Cure of the PAA films adhered on a substrate induced the spontaneous orientation even if the films were considerably thick (similar to 50 mu m); in contrast to that, the cure of the freestanding thick film did not. However, upon cure of the free-standing thin films (similar to 10 mu m), the spontaneous orientation behavior was observed. For rigid PI systems in which interchain stacking preferentially occurs, thermal cure of the PAA films on a substrate forms liquid-crystal-like highly oriented regions, and simultaneously, apparent stretching (due to constraint of film contraction during imidization) promotes the molecular orientation further. The mechanism is closely associated with a "cooperative effect" in which the neighboring chains enhance the molecular orientation of each other during cure. In addition, structural changes (orientations of the chain axis and a molecular plane and molecular packing) upon stepwise annealing were followed. Polarized infrared absorption spectra measured at an incidence angle demonstrated that the phthalimide molecular plane in the PI film on a silicon wafer aligns somewhat parallel to the film plane, but no significant orientational and conformational changes occurred upon stepwise annealing.