Fatigue loading tests have been carried out to investigate the fatigue properties of hybrid beams with steel and concrete in marine environments. The test beams were two kinds of open sandwich beam; a steel plate was installed on the tensile surface of concrete and one king of sandwich beam: two steel plates were installed both on the tensile and the compressive sides of concrete. The total number of specimens was 18.
　The hybrid beams were broken with the tensile fracture of the steel plate at the point of welding. When upper loading limit was just about 20% of the ultimate load of the beam, the beam did not break after 2 million cycles of loading. On the basis of the experimental results, the empirical S-N curve on the steel plate where plenty of shape steel and reinforcing bars were welded was proposed. The sum of the damage contributions was smaller than 1.0 in the tensile steel plate subject to waves around Japan using Miner's rule and the proposed S-N curve. Therefore, the safety against fatigue failure of the hybrid beam was confirmed.