As a metast able pha se, poly (3-butylthiophe ne) (P3BT) form II is generally obtaine d by solvent vapor treatment in rigorous condi tions and it is easi ly transformed to more comm only obs erved form I with heating. In the present study, by low temperature agi ng in carbo n disulfi de (CS₂) solution, a simple and feasible method was proposed to prepare neat P3BT form II. Thus prepared crystal exhibits the distinct bundle morphology composed of single-crystal-like nanofibers. With such sample, the transitional mechanism from form II to form I modification is further investigated by in situ WAXD combined with FTIR spectroscopy. It is found that form II transfers to form I via a quasi-ordered smectic phase rather than a direct solid-to-solid transition. This study could be helpful for controlling the crystal structure and morphology in P3BT thin film, which is closely related to its performance in electronic devices.