The degrees of uniaxial and in-plane molecular orientation for homopolyimides (PI) and PI/PI binary blends were estimated from the absorption dichroism of perylenetetracarboxydiimide (PEDI) bound to the main chains. Thermal cure of a slightly drawn film of poly(amic acid) (PAA) derived from 3,4,3,′4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA), BPDA/PDA, showed marked spontaneous orientation toward the stretching direction. The dependence on the chain structure revealed that in addition to the chain stiffness (linearity) the molecular packing is an important factor for the spontaneous orientation behavior. In a miscible binary blend system composed of semirigid BPDA/ PDA labeled with PEDI and flexible BPDA/ODA (ODA: 4,4′-oxydianiline), the extent of spontaneous orientation for the labeled BPDA/PDA chains decreased with increasing BPDA/ODA content in the blends. From the result, the marked spontaneous orientation behavior of homo-BPDA/PDA is interpreted as a kind of "cooperative phenomenon" where the neighboring chains promote the molecular orientation of each other in the cure process. Effects of film thickness, amount of residual solvent, cure method, and heating rate on the extent of spontaneous orientation were also discussed for the homo-BPDA/PDA system.