As a metastable phase, poly (3-butylthiophene) (P3BT) form II is generally obtained by solvent vapor treatment in rigorous conditions and it is easily transformed to more commonly observed form I with heating. In the present study, by low temperature aging in carbon disulfide (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. (C) 2016 Elsevier Ltd. All rights reserved.