Regioregular poly(3-hexyl thiophene) (RR P3HT) is drop-cast to fabricate field-effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well-dispersed nanofibrils to well-developed spherulites by changing solution processing conditions. The morphological correlation with the charge-carrier mobility in RR P3HT thin-film transistor (TFT) devices is investigated. The TFT devices show charge-carrier mobilities in the range of 10–4 ∼ 10–2 cm2 V–1 s–1 depending on the solvent used, although grazing-incidence X-ray diffraction (GIXD) reveals that all films develop the same π–π-stacking orientation, where the <100>-axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>-axis orientation distribution of the π–π-stacking plane with respect to the film normal play important roles on the charge-carrier mobility of RR P3HT for TFT applications.