Poly (3-butylthiophene) (P3BT) is a much less studied conjugated polymer despite its high crystallizability and thus excellent electrical property. In this work, morphology of P3BT at different crystalline polymorphs and solvent/thermal induced phase transition between form I and U modifications have been intensively investigated by using optical microscopy, electron microscopy, differential scanning calorimetry, and X-ray diffraction. It is shown that a direct deposition from carbon disulfide (CS2) at fast evaporation results in P3BT crystals in form I modification, giving typical whiskerlike morphology. In contrast, low evaporation rate from CS, leads to formation of form II crystals with spherulitic morphology, which is so far scarcely observed in polythiophene. We further show the crystalline transition of P3BT crystals from form I to II modification upon controlled solvent vapor treatment (C-SVT), in which both the transition rate and the size of the spherulite obtained depend on the vapor pressure and treatment time, respectively. We ascribe the driving force for this transition to the stability difference of these two modifications in CS2. However, form II transfers reversely back to form I at about 159 degrees C via a solid-solid phase transition at elevated temperature. The transition behaviors of P3BT crystals upon both solvent vapor treatment and thermal heating at its bulk state reveals that form II instead of far more commonly observed form I modification is the thermodynamically stable structure for P3BT at room temperature.