Crystallization behaviors on two-step isothermal crystallization at T-c1 and T-c2 (< T-c1) were examined by fast scanning calorimetry for poly(butylene terephthalate) in order to identify the dominant process during the secondary stage of crystallization at T-c1 after the completion of spherulitic growth under isothermal condition. From the difference between the melting peaks obtained before and after the low-temperature crystallization step at T-c2, the distinction of the following two processes became realizable: the growth of crystals formed at T(c2 )and the stabilization of crystals formed at T-c1. The dependences on T-c2 and the duration of times Delta t(c1) and Delta t(c2) revealed the following: The formation and growth of new crystals are controlled by thermodynamic driving force determined by supercooling, and hence are suppressed at T-c1 higher than T-c2. The crystal stabilization is controlled by the mobility of polymer chains, and became more pronounced at T-c1. Therefore, the dominant process of the secondary crystallization under isothermal condition at T-c1 should be the stabilization of crystals formed in the primary stage and the formation of new crystals will only have a secondary effect.