In a context of reduction of greenhouse gas emission, the development of hydrogen as an energy carrier implies the transport of large quantity of hydrogen gas over long distances. The option of using the existing Natural Gas (NG) network raises the question of the amount of hydrogen gas that can be blended with natural gas while ensuring safe operating conditions for the pipe, especially in presence of crack-like defects. With this respect, the present study was undertaken in order to get a first assessment of the impact of the hydrogen gas content in natural gas on the crack growth resistance of pipeline girth welds. With this aim, fracture toughness and fatigue crack propagation tests were carried out at RT on a servo-hydraulic machine equipped with a pressure vessel. The CT samples used in these tests were extracted from different actual circular welds (respectively 350 and 900 mm pipe diameter, 7 and 13 mm thickness) representative of those present in the network so that the crack plane is located in the joint (Weld Centre Line so far). Two couples of steel and welding process were studied, namely a modern pipe steel L485 (X70) welded by mechanized welding process (GMAW) and a vintage pipe (X60) carbon steel with a manual cellulosic electrode process (SMAW). Two environmental conditions were considered so far, namely representative NG including 9 minor species and a 25%H -NG blend with a total pressure of 8.5 MPa. The fracture toughness for both types of welds results indicate no significant effect of hydrogen on the K values, while the plastic opening and the CTOD values are affected at different degrees. Besides, fatigue crack growth rates are enhanced by more than one order of magnitude in the 25%H -NG mixture in the high ΔK region as compared to NG. Fractographic observations are presented to support these results.