Kinetics of II to I polymorphic transformation in isotactic polybutene-1 (PB-1) and its annealing temperature and time dependencies have been investigated by means of differential scanning calorimetry (DSC) and in-situ wide angle X-ray diffraction (WAXD) techniques. The PB-1 samples were isothermally crystallized into metastable form II crystalline modification and annealed at a lower temperature (Tl) followed by annealing at a higher temperature (Th) or annealed at a single temperature (Ts) to promote polymorphic transition from form II to I. This solid-to-solid phase transition is shown to be a two-step process including nucleation and growth suggested by the result that more form I was obtained after being annealed at Tl and Th than annealed at Ts for same period. Annealing at Tl benefits nucleation due to internal stress induced by difference of thermal expansion coefficient between the amorphous and crystalline phases, while annealing at Th is beneficial to growth owing to rapid segmental diffusion. At a given annealing time at Tl (tl) and at Th (th), and fixing one of temperatures between Tl and Th, it shows a maximum in the transformation-temperature profile that can be correlated with the optimal temperature for nucleation or growth. The phase transition was efficiently accelerated with the increase of lamellar thickness, and appears a linearly relationship. Our results decomposed the polymorphic transition into nucleation and growth for the first time and provided a simple and effective way for rapid transition of form II to I in PB-1.