We evaluated the crystallization effect of an internal electric field on thaumatin by application of extremely low voltage. A transparent cell used for crystallization comprises two parallel-plate electrodes of indium tin oxide (ITO) films, which enables the observation of the protein crystal growth through the plate electrodes. In suspensions with a precipitant, thaumatin crystal growth was induced by applying a sine-wave voltage of 1.06 V at f=20 Hz for 10 hr. We confirmed that low applied voltage promoted crystallization for thaumatin.