Poly(trimethylene terephthalate) (PTT), a highly birefringent aromatic polyester, has been investigated to explore the correlation between crystal morphology, optical birefringence properties, and crystallization mechanism. PTT exhibits large and well-defined ring-banded spherulites which make it a good candidate for the study of polymer crystallization and self-assembly. Optical interference colors that ornament the morphological image of PTT crystal under POM are found to be strongly related not only to the sample thickness, but also to the lamellar structure and arrangement within the spherulites. Besides, banding structures of the spherulites are also found to be varied with the curvature of the initial nucleus. A novel crystallization mechanism of PTT has been built from comprehensive analyses of morphology, optical colors, and lamellar assembly approaches.