[en] A core design of a high conversion type innovative water reactor for flexible fuel cycle (FLWR) with thermal output of 3926 MW has been constructed. FLWR mainly based on established light water reactor (LWR) technology has been investigated by the Japan Atomic Energy Agency (JAEA) in order to ensure sustainable energy supply in the future. This FLWR is a boiling water reactor (BWR) with a tight triangular fuel rod lattice and the uranium plutonium mixed oxide (MOX) fuel. FLWR is designed for two types of cores to be developed in succession. The preceding core is a high conversion type FLWR (HC-FLWR) and a design study of its has been performed. The design study consists of two steps of analyses. The first step was preliminary parametric survey calculations with a neutronics calculation code and simplified calculation models to find adequate core specification range. From this calculation, the fissile plutonium (Puf) enrichment range of MOX fuel of 9-11% and the diameter of the fuel rod of 1.12 cm were obtained. The second step was more detailed calculations with a nuclear and thermal-hydraulic coupled calculation code MOSRA and one-dimensional and three-dimensional calculation models. Through the one-dimensional core burnup calculations, the following design values were obtained. The Puf enrichment of MOX fuel is 11%. The heights of upper blanket, MOX and lower blanket regions are 5 cm, 85 cm and 5 cm, respectively. With these values, three-dimensional nuclear and thermal-hydraulic coupled core burnup calculations were performed. In this analysis, effects of the fuel loading pattern were also investigated. Finally, a neutronics design of HC-FLWR core with a negative void reactivity coefficient, a conversion ratio of 0.84 and a discharged burnup of 56 GWd/t was obtained. (author)