The effect of solvent evaporation rate on the crystal modification and the thermal stability of solution cast regioregular poly-(3-butylthiophene) (P3BT) film was investigated by WAXD, DSC, and FTIR It is found that, independent of the solvent evaporation rates, the P3BT films prepared from chloroform solution demonstrate essentially the same interlayer distance, which is very close to the reported value of form rather than the conventional form I. However, in the DSC heating curves, it is interesting to find that a single "melting" peak and a double one in the temperature region of 45-100 degrees C can be observed respectively for P3BT films prepared with fast and extremely slow solvent evaporation rates. The double melting behavior of P3BT in this low-temperature region has never been discussed in the literature. By temperature-dependent infrared spectroscopy, it is shown that these low-temperature "melting" peaks are related to the order-to-disorder phase transition (form I' to form I) of P3BT. Furthermore, our spectral studies reveal that the "melting" peak around 58 degrees C in the DSC heating trace is associated with the conformational disordering of butyl side chains, whereas the destruction of backbone planarity and the disordering of pi-pi stacking can take place at higher temperature (ca. 82 degrees C) with decreasing the solvent evaporation rate.