The sea tests of quick start and rapid load increase were carried out on several types of naval steam turbines by their manufacturers in 1959 and 1960. In the tests, the turbines were acceralated from 12 knots speed to the maximum speed within 25-14 minutes. The temperature of turbine casings and the relative expansion of the casings and the rotors were measured. This paper reports mainly the thermal stress of the horizontal flanges of the high pressure turbine casings, which seemed to be the most important in rapid load increase. The authors conculude that; 1) Marine steam turbines are rather tough in rapid load change than usually considered. 2) The heat stress at the horizontal flanges of the casings becomes the first limitation against rapid load increse in marine turbines of ordinary design. 3) The warming-up time of a few hours is completely unnecessary, and could be saved to several minutes without any trouble even in turbines of ordinary design.