Solvent extraction of Np with n -octyl ( pheny1) -N, N -diisobutylcarbamoylmethylphosphine oxide (CMPO) from HNO3solution containing H2O2was studied from the aspect of extraction rate. Neptunium initially in the pentavalent oxidation state can be extracted by CMPO even without the addition any redox agents, but the distribution is rather low. The H2O2was found to enhance the distribution ratio. The extraction rate was almost proportional to the 1.5-power of H2O2concentration. The dependence of the extraction rate on HNO3concentration was also examined. Reduction of Np(V) with H2O2occurs rapidly in the presence of CMPO. The extraction rate is not strongly affected by the presence of tri-n-butylphosphate in the organic phase. As a diluent, decalin gives a greater extraction rate than n-dodecane.