A pressboard consisting of an angle ring and a cap is installed at the primary and secondary coil windings of a transformer. This pressboard is related to transformer durability. Since transformer capacities are increasing, it is necessary to develop a pressboard of high voltage grade (above 150 kV). In this study, a press-forming technology was developed for the pressboard of a 154 kV transformer. To determine the optimum shape of the pressboard, the press-forming factors were experimentally evaluated through the mechanical tensile strength and AC withstand voltage tests under wet conditions. From the results, the optimum size and shape of the pressboard was found to be R15~R20 mm under a forming pressure of 10 MPa. When the relative humidity (RH) was less than 50%, the moisture absorption rate in oil decreased by 25~40% compared to that in air. However, when RH exceeded 50%, the difference in the moisture absorption rate between the cases of oil and air was just 8~10%. The AC withstand voltage of the pressboard was weak against moisture. The average AC withstand voltage at $40^{circ}C$ and RH of less than 50% decreased by about 15% compared to the dried condition, and the maximum value decreased by 30% at $40^{circ}C$ and RH of more than 50%.