The effective capture cross section (σ) of 238Np for thermal neutrons has been measured as data for the burn-up analysis of irradiated fuels and for the study of nuclear transmutation of minor actinides. A sample of 237Np was irradiated for 10 hours at the Kyoto University Reactor. Neptunium-238 and -239 were produced simultaneously via the double neutron capture reactions 237Np(n, γ)238Np(n, γ)239Np. The neutron flux at the irradiation position was monitored using Au and Co wires. The epithermal index r√T/T 0 in Westcott's convention was deduced as 0.03. To measure the activities of 238Np and 239Np, a high-purity Ge detector with a BGO (Bi4Ge3O12) anti-coincidence shield has been developed. The σ of 238Np was deduced from the ratio of these activities. The result obtained is 479± 24 b for the σ of 238Np. The result is compared with data in the evaluated nuclear data libraries, ENDL-86, ENDF/B-VI, and JENDL-3.3.