Effects of humidity and pre-discharges on repetitive partial discharge inception voltage (RPDIV) with twisted enameled wires under bipolar and unipolar impulse voltages have been investigated. The RPDIV under the bipolar impulse voltages have become relatively scattered during the early stage of the voltage sequence in the dry condition. After sufficient number of the voltage sequence, it has reached to a stable state. On the other hand, the RPDIV have reached to a stable state from the beginning of measurement in the humid condition. In the case of the unipolar impulse voltages, no scattered state of the RPDIV has been observed. Surface potential on the enameled wire after the discharge has been measured with a surface potential meter in order to clarify effects of pre-discharges on the RPDIV. As the results, the fluctuation in the RPDIV is associated with the accumulation of the surface charges generated by the discharges.