This research work is focused on the hydrogen embrittlement (HE) effect on mechanical properties of X-120 microalloyed steel and to determine the hydrogen permeability parameters after tempering treatment at 200, 400, and 600 degrees C for 10 min. Stress-strain curves were obtained in as-received and tempered conditions, and permeability tests were carried out to determine the HE susceptibility. The results showed a microstructure composed of martensite-bainite, acicular-ferrite and retained austenite with no appreciable change on its morphology after tempering treatment. However, the microalloyed steel showed a decrease in its mechanical properties after charging conditions as tempering temperature increases. Fractography analysis revealed a decohesion and dislocation emission mechanism of HE which induce nucleation and crack growth. The index embrittlement (%EI) was related to the reduction in effective diffusion (D-eff) and increment of apparent concentration (C-app) due to the increment of irreversible traps as a function of the type of precipitates presented in tempering stages. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.