In order to investigate the low-cycle fatigue performance of plates under hydrostatic pressure, four cubic boxes with 600 mm of each side were fabricated from 20 mm thick plates of Q & T high tensile strength steel (HT 70), and internally pressurized cyclically at peak nominal stresses comparable to the yield strength of the material. From these experiments the following conclusions are obtained : - (1) Cracks initiate at the toe of weldment of corner joints inside of the box, at about 1, 500-4, 000 cycles. (2) Number of cycles for crack initiation and average crack propagation rates are related to total strain range. (3) Weld contouring on the corner joints by grinding has little effect on the cycles for crack initiation, however, reduces the possibility of crack initiation. (4) Boxes made from high notch-tough materials begin to leak when cracks propagate through the plate thickness, and no unstable fracture will occur.