Polymer conformation is the molecular basis underlying essentially all physical properties of polymers, and propensities of chain conformation and conformational energy play central roles in crystalline structure formations and structure transitions of polymers. Isotactic polybutene-1 (iPB-1) is a semi-crystalline polymer with polymorph and puzzle-like structure transitions. The stable form I of iPB-1 with excellent physical and mechanical properties can hardly be obtained directly from the melt but is formed from unstable form II which would spontaneously and slowly transform into form I. To obtain form I directly from the bulk iPB-1 melt by bypassing the unstable form II formation or ultrafast transition from form II is a challenging task. Here, we have investigated the formation and transition of iPB-1 crystalline structures with POM, DSC, in situ X-ray scattering and FTIR. The temperature related transition of form II to form I with or without solvents is characterized. It is found that the rate of transition decreases with temperature. An unexpected ultrafast transition is observed with solvents at room temperature, but the “stable” form II is investigated at high temperature with and without solvents. The conformation based mechanism of form II formation and packed-mesophase based transition is proposed, which would be helpful in understanding the physics of iPB-1 and polymer crystallization.