Field-effect transistors were prepared with magnesium phthalocyanine (MgPc) and vanadium pentoxide (V2O5). Vapor treatments of several organic solvents were carried out for MgPc thin films. The UV-vis absorption spectra, surface morphologies, X-ray diffraction patterns were largely changed with the treatments. The FET characteristics were also improved by the treatments and dichloromethane treatment induced the largest improvement. Furthermore, V2O5 film, which is used as carrier generation layer in organic light-emitting diodes, were inserted in the FET and the FET characteristic was changed. The vapor treatment and V2O5 insertion should be used for controlling FET performance.