The morphology of poly(4-phthalimide) (PPI) crystals was examined by using reaction-induced crystallization of oligomers during solution polymerization. Polymerizations of alkyl 4-aminophthalates such as I-ethyl 4-aminophthalate (1EAP), 2-ethyl 4-aminophthalate, 1-hexyl 4-aminophthalate, and 2-hexyl 4-aminophthalate were carried out at 330 degrees C in a mixture of isomers of dibenzyltoluene without stirring. Nanoscale PPI ribbons having smooth surface were formed by the polymerizations of these monomers. The ribbons prepared from 1EAP at a concentration of 2.0% for 24 h were averagely 12 mu m in length, 150 urn in width, and 8 nm in thickness, in which the molecular chains aligned along the long axis of the ribbon. The phase-separated oligomers were fully cyclized imide oligomers, and this is of great importance for the formation of the ribbon. The obtained ribbons possessed high crystallinity and exhibited excellent thermal stability.