Polybutene-1 (PB-1) is an important polyolefin showing superior creep resistance even at elevated temperatures. However, PB-1 crystallizes always into a metastable form II when being cooled down from the molten state due to the lower nucleation barrier, and form II crystals would transform spontaneously into stable form I. The direct formation of form I from the melt possesses obviously technical importance as well as scientific significance. It was observed that form I’ was directly crystallized under CO2 pressure higher than a critical value, Pc, which varies with crystallization temperature. One could conclude that the crystallization of form II was hindered while that of form I was accelerated. Solid-state NMR results suggested that crystalline stems in form II performed uniaxial rotational diffusion accompanying side-chain conformational transitions while crystalline stems and side-chain conformations of the most stable form I were completely fixed. Under high pressure CO2, the polymer chain flexibility was highly enhanced. The increase of chain mobility exhibits a negative effect on the deposition of chain segments onto the form II crystal growth front due to a rapid relaxation of the chains into amorphous state, but has a positive effect on crystal growth of form I. Once the chain segments were deposited onto form I crystal growth front, it was expected to be fixed and difficult to escape from the form I crystal phase.