The purpose of the present work is to study the effect of the hydrogen content in a H₂-NG (Natural Gas) blend on the mechanical behavior of a C-Mn low alloy steel L485-MB (NF EN ISO 3183) under monotonic loading. Different testing conditions were explored for the blend: 2% H₂-NG, 25% H₂-NG, 100% H₂ and 100% NG at a total pressure of 8.5 MPa using two slow strain rates: 10⁻⁴ s⁻¹ and 10⁻⁶ s⁻¹. Tests in the blend were compared to those under pure hydrogen at the same partial pressures. Furthermore, two surface roughnesses have been explored, one corresponding to a mirror polish, the other as lathed. The embrittlement has been assessed by necking measurements and fracture analysis for the whole testing conditions. The results show a drop of ductility with the increase of partial pressure of hydrogen in NG. Besides, no significant effect of hydrogen were observed on flow stress. In addition, the L485-MB pipeline steel presents a shear type fracture mode in high partial pressure of hydrogen, whereas a cup and cone type fracture were observed in air, NG and low partial pressure of hydrogen in NG (0.2 MPa H₂).