Organic field-effect transitors (OFETs) based on regio-regular-poly(3-hexylthiophene) (P3HT) have been studied as a function of the amount of impurities in the active polymer. P3HT have been synthesized and successively purified. Transistors based on each fraction have been manufactured on a polyimide dielectric following a procedure described by Nunzi and co. The configuration is bottom-gate of ITO coated glass and evaporated gold top drain and source electrodes. The channel is 3 mm wide and 40 µm long.P3HT fractions have been analyzed by RBS and PIXE spectroscopies to show the presence of remaining monomers, reactants or catalysers. Impurities such as Ni, Cl, Mg, Ca, Fe and Zn have been detected in non-fully purified P3HTs. The final product (F4) is free of impurities. We demonstrate that such impurities have a significant effect on OFETs. The on/off ratio is strongly affected. The current at 0 V gate bias strongly increases as a function of impurities. Because saturation of OFETs vary as a function of impurities, the mobility has been extracted from the linear regime of characteristics and is found slightly dependent on the impurity content. It is believed that defects act as dopant moities since similar mobility dependence has already been observed in the case of electrochemically doped-P3HT by Jiang et al