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A mechanistic aspect of the susceptibility to the delayed fracture wasstudied with an emphasis on the critical behaviour of the subsurface growth ofQuasi-Cleavage (QC) and Inter-Granular (IG) cracks. The materials employed were0.35%C plain carbon steel S35C and boron added bolt steel Bolten110N which werequenched and tempered to have various levels of yield strength ranging from500~1400MPa. These were put into sustained load fracture test with cathodic hydrogencharging. The delayed fracture strength was evaluated by the threshold stress (σth) atthe elapsed time of 104 minutes. Fractographic analysis shows us that QC-IG-MVC(Micro-Void-Coalescence) cracking process can be essential aspects in the delayedfracture of steels. A low susceptibility to delayed fracture in low strength steels can beexplained by the absence of IG crack in the crack growth process where the crucialblunting occurred at the crack tip.