A criticality safety study on a light water moderated and reflected coupled core loaded with highly enriched uranium fuel was performed in the Kyoto University Critical Assembly. The critical mass and neutron flux distribution were measured systematically as a function of the separation distance between the two cores, varying the H/235U atomic ratio (i.e., the moderator-to-fuel volume ratio). These data were analyzed with the SRAC code system to assess the capability of diffusion theory to analyze the coupled-core system. It was found that the critical mass of the coupled core showed the minimum when the two cores were separated by a certain distance depending on the neutron spectrum in the core region. The neutron flux peak value at the water gap region reached the maximum when the separation distance was 5 to 6 cm. The results calculated with the diffusion code installed in the SRAC system agreed well with the experimental data.