Extraction of Np with di-isodecyl phosphoric acid (DIDPA) from HN03 solution containing H2O2>was studied from the aspect of the extraction mechanism and the enhancement of the extraction rate. Neptunium initially in the pentavalent oxidation state can be extracted into DIDPA even without any redox agents, but the extraction rate is rather slow. In the present work, H2O2was found to accelerate the extraction. The rate was almost proportional to the 1/2-power of H2O2concentration. Dependence of the extraction rate on HN03 concentration was also examined.