The effects of the prior austenite grain sizes in hydrogen embrittlement of Co-containing 18Ni 300 maraging steel were studied employing Slow Strain Rate Testing (SSRT) in 0.6 M NaCl electrolyte under simultaneuos cathodic polarization. The material was susceptible to hydrogen embrittlement in all investigated conditions. In addition, the examination of the fractured surface revealed that the presence of hydrogen in steel promotes the formation of quasi-cleavage regions and hydrogen-induced cracks along the grain boundaries. However, the refining of the prior austenite grain allowed an improvement in the HE resistance. Moreover, EBSD analysis showed that intergranular cracks propagated along to grain boundaries orientated to {001} planes parallel to normal direction, whereas they were deflected on {101} and {111} crystallographic planes. •The control of the prior austenite grain leads to the improvement of the HE resistance.•Intergranular cracks propagated along the grain boundaries orientated to {001} planes.•The cracks were deflected on grain boundaries {110} and {111} crystallographic planes.