Using finite element method (FEM) calculations and a new design of permeation testing under tensile loading until fracture we revisit the question of hydrogen embrittlement of martensitic steel. More precisely, we explore the impact of mobile and trapped hydrogen on ductile and brittle fracture of martensitic steel using local approach of fracture and a specific analysis of the defect evolution under hydrogen flux (vacancies and dislocations). We discussed damage conditions in relation with mechanical state using tensile samples with different notches for samples strengthening under hydrogen flux. We highlight that the mobile hydrogen conduces to a quasi-cleavage fracture mode at a scale of martensitic laths. Stress-strain and hardening curves are less affected by hydrogen; in opposite the viscoplastic behaviour is dependent on the mobile hydrogen. This last result is discussed in term of edge and screw dislocations mobility using multi-relaxation mechanical test and apparent activation volume determination.