Abstract The influence of hydrogen on strength and failure of dissimilar weld joint of the two creep resistent steels with different chemical composition (9Cr-1Mo,V and 0,5Cr-0,5Mo,V) welded by arc welding after tensile strain tests at ambient temperature (+20 °C) was investigated. Hydrogen presence in weld joint decreases notch strength (Rmv) and reduction area (Zv) in both the heat affected zone (HAZ) and welding metal (WM) at tensile straining (+20 °C) in the following order: 1. HAZ 0,5Cr-0,5Mo,V (STN15 128), 2. Weld metal T24 (2,5Cr-0,5Mo,V,Ti,B), 3. HAZ 9Cr-1Mo,V (P91). Changes in appearance of fracture modes and mechanism of failure, induced by presence of hydrogen in dissimilar weld joint by means of fractography analyses of fracture surfaces of the HAZ and WM were observed. Ductile failure by dimpling at absence of the hydrogen changes to brittle cleavage failure caused by presence of hydrogen. It was confirmed that nucleation of voids in tempered martensite steels at usual tensile load conditions predominantly occurs by separation of carbide/matrix interfaces. Voids initiate and grow preferentially around the large particles, later their coalescence with the voids around small particles between the crack tip and large voids leads to the final failure consisting of the large and small dimples. The hydrogen in weld joint initiates a “brittle fracture” and cleavage (quasi-cleavage) on the carbide/matrix interfaces and decreases energy of the crack propagation. The mode of failure of the weld joint after hydrogen charging depends on the microstructure, size and distribution of particles (carbides) in different parts of the weld joint (HAZ, WM).