A new approach for evaluating the apparent equilibrium and rate constants for the reaction of Np(V) with nitric acid by using the process data of multistage countercurrent solvent extraction experiments is proposed. The numerical simulation of neptunium extraction in the co-decontamination process of the Purex process is carried out on the assumption of the rate equation. The apparent equilibrium and rate constants are determined by the nonlinear least-squares method with experimental data for multistage countercurrent solvent extraction with a numerical simulation code. The determined apparent equilibrium and rate constants are 1.25 to 1.97 x 10-2 min-1 and 6.04 to 11.5 x 10-4 (ℓ/mol)3/2, respectively. These values are consistent with those obtained in other kinetic studies.