The effect of aged structure on the susceptibility to hydrogen embrittlement (SHE) of a 13Ni-15Co-10Mo 2750 MPa grade maraging steel has been investigated for the specimens thermomechanically processed and then aged at various temperatures. In particular, the objective of this study is to elucidate whether the stage of aged structure is or is not severely susceptible to hydrogen embrittlement, when aged at 500°C to produce the maximum strength. The SHE was evaluated using the strain rate dependence of reduction of area in tensile tests in air. The SHE increased markedly when aged at temperatures below 475°C. The aged structure to produce the maximum strength at 500°C showed obviously the same of aged structure with the lower SHE. The higher SHE of this steel compared with the lower strength maraging steels, therefore, could not be attributable to the stage of the aged structure. The increase of SHE by aging at relatively low temperatures was discussed in terms of the both changes in slip mode and solubility and diffusivity of hydrogen due to the presence of the fine and coherent precipitates.