The quantum-size state (QSS) of semimetal Bi films has been studied by electric field effect (EFE), namely, the change of the conductance at the electrical charging of the film. The ratio of the electron and hole density of states g n /g p has been measured by EFE . The thickness dependence of this ratio exhibits a characteristic oscillation pattern, which is an unique label of QSS. This method allows a rather precise determination of the film thickness period of the oscillations. The result is in excellent agreement with the value derived from the dispersion law of bulk Bi.