The thermal-neutron capture cross-section (sigma(0)) and resonance integral (I-0) were measured for the Np-237(n,) Np-238 reaction by an activation method. A method with a Gadolinium filter, which is similar to the Cadmium difference method, was used to measure the sigma(0) with paying attention to the first resonance at 0.489eV of Np-237, and a value of 0.133eV was taken as a cut-off energy. Neptunium-237 samples were irradiated at the pneumatic tube of the Kyoto University Research Reactor in Institute for Integral Radiation and Nuclear Science, Kyoto University. Wires of Co/Al and Au/Al alloys were used as monitors to determine thermal-neutron fluxes and epi-thermal Westcott's indices at an irradiation position. A -ray spectroscopy was used to measure activities of Np-237, Np-238 and neutron monitors. On the basis of Westcott's convention, the sigma(0) and I-0 values were derived as 186.9 +/- 6.2 barn, and 1009 +/- 90 barn, respectively.