Abstract: Currently, it is most economical to transport hydrogen with the existing natural gas pipelines by blending it into the natural gas among the storage and transportation technologies of hydrogen energy, and thus the transportation technology of hydrogen-blended natural gas is widely concerned by research institutes at home and abroad. However, hydrogen embrittlement may be caused to the pipeline steel due to the contact of hydrogen with base metal during hydrogen transportation, which further leads to the deterioration of mechanical properties of pipeline steel, threatening the safe operation of pipelines. Therefore, the change law of the mechanical properties of X52 and X80 steel typically used for natural gas pipelines under the actual hydrogen-blended conditions was studied through in-situ tensile testing and fracture morphology analysis under the environment of high-pressure gas-phase hydrogen. Meanwhile, the influence of hydrogen partial pressure on yield strength, tensile strength, percentage reduction of area and hydrogen embrittlement index of material was analyzed. The study results show that, the ductility of X52 and X80 steel is declined and the hydrogen embrittlement is intensified with the increasing of hydrogen partial pressure. Compared with X80 steel, X52 steel is more suitable for the hydrogen-blended natural gas transportation. Generally, the study results could provide reference to the material selection and safe operation of hydrogen-blended natural gas pipelines in the future.