The competitive growth of form II and form I in isotactic polybutene-1(PB-1) was investigated with fast scanning chip calorimetry. By systematically varying the isothermal time, the pre-crystallized PB-1 samples with different crystallinity of form II are obtained and then used to study the competition between the growth of form II and the transformation of form II into form I. All the samples were annealed at 258 K for 60s to help nucleation of form I so that fast transformation could be performed. Crystal transformation kinetics results indicated that the competitive growth of form II and form I can be classified into three regimes. When the crystallinity of form II is small enough, fast transformation from form II to form I does not take place, attributed to lacking of the nuclei of form I as the degree of internal stress is extremely low. This suggests that only certain strength of internal stress and certain crystallinity of form II enable the nucleation of form I and then promote the transformation from form II to form I. With increasing crystallinity of form II, both fast transformation and slow transformation are observed. When crystallinity of form II further increases, only fast transformation is observed. However, the transition rate decreases and then reaches a plateau, resulting from the rearrangement of polymer chains and the increasing regularity of form II, indicating that the increasing regularity of form II inhibits the transformation from form II to form I.