Rubber O-ring seals have been extensively used in high-pressure hydrogen storage systems for preventing gas leakage. It is important for the design of rubber O-ring seals to clarify the sealing characteristics of the rubber O-ring under high-pressure gaseous hydrogen. In this paper, a finite element analysis model accounting for swelling due to dissolved hydrogen was developed to investigate the sealing performance of the combined seal structure constructed of a rubber O-ring and a wedge-ring under ultrahigh pressure gaseous hydrogen. The effects of wedge-ring, hydrogen pressure, and swelling on the sealing capacity of the combined seal were analyzed. Consequencely, it was found that the wedge ring by itself can prevent the O-ring from extruding under ultrahigh pressure by filling up the clearance between the grooves, and the wedge-ring with the angle of 45 degrees was the most suitable in comparison to those of 30 degrees and 60 degrees. The combined seal structure can enhance automatically the sealing capacity with increasing hydrogen pressure, and the increase of the peak contact stress was linearly proportional to that of the fluid pressure. In addition, the rising peak contact stress induced by swelling due to dissolved hydrogen could result in an improvement in sealing capacity, while the swelling may accelerate the mechanical damage of the O-ring owing to an increase in Mises stress. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.