Hydrogen induced cracking (HIC) behavior of X120 pipeline steel with different amounts of Mn and Al was investigated using NACE standard TM0284-2005 and back-scattered electron imaging (BSE). The results demonstrated that inclusions and microstructure play significant roles in HIC susceptibility in X120 steels. The more the area and volume fraction of inclusions in the steel, the more the steel is susceptible to HIC. Three types of inclusions exist in the tested X120 steel and they play different roles in cracking. The critical size resulting in HIC cracks for pure MnS inclusions or mixed inclusions (inclusion clusters mainly composed of two or three different phases and compounds) of Mn/Al ratio >1 is smaller than that of pure Al2O3 inclusions or mixed inclusions of Mn/Al ratio <1. No HIC crack was observed at inclusions enriched in Si. The microstructure containing lath bainite and martensite/austenite (M/A) microconstituents is more susceptible to HIC than that with granular bainite and ferrite.