The crystallization mechanism of “one-stage route” or “two-stage route” of poly (butylene naphthalate) (PBN) can occur at different cooling conditions, which lead to the formation of different kinds of crystal forms and morphologies. In this work, differential scanning calorimetry and fast-scanning chip-calorimetry were employed to investigate the crystallization behavior of the melt of PBN. Moreover, the resulting crystal forms and morphologies of the melt of PBN at different degrees of supercooling were observed using polarized-light optical microscopy and atomic force microscopy. The results indicated that the crystallization mechanism of PBN shifts from “one-stage route” to “two-stage route” with the increase of cooling rates and the critical cooling rate is about 40 K/s. Besides, β’- and α-crystals with lamellae structure formed at low supercooling of the melt, while small domains formed at high supercooling. Meanwhile, it was confirmed that the transformation from liquid crystalline mesophase into α-crystals is a solid-solid phase transformation.