Naval boilers of two destroyers were tested. The boilers were of a same design and manufactured by two different manufacturers, each planned and carried out the tests. The boilers were steamed up to their working pressure within 100-50 minutes, and the temperature of the drums, the superheater tube walls, and the combustion gas was measured. The deformation of the water drum was also measured and it was of considerable amount, for the water near the bottom of the drum remained cold even when the steam pressure reached to its working state. The stress of the water drum was measured using high temperature strain gauges, and the stress value was found nearly critical in 50 minutes steaming-up tests, which might be the first limitation in rapid steaming-up. The superheater tube walls reached to the maximum value soon after the steam started, and decreased gradually as the steam flow became more than 1-2t/h. Uneven distribution of gas flow and consequently of tube wall temperature is also mentioned.