The experiment for the sympathetic detonation (Sudo et al., 1951) (Fukuyama et al., 1958) in water was conducted. Composition B (RDX: 64%, TNT: 36%, Detonation velocity: 7900m/s) was used for both donor (the thickness was 50mm, and the diameter was 31mm) and receptor charges. The distance between the donor and the receptor, and the thickness (5, 7.5, 10mm) of the receptor were varied in the experiments. In order to investigate the basic characteristics of the underwater sympathetic detonation of high explosive, the sympathetic detonation phenomena were visualized by a high-speed camera (HADLAND PHOTONICS, IMACON790) in forms of streak and framing photographs. The 200ns/mm streak velocity was 2㎲. Manganin gauges (KYOWA Electronic INSTRUMENTS CO. SKF-21725) were used for the pressure measurements. The gauges were set under the receptor. The pressures during the complete and incomplete explosions were measured.