Abstract: The mechanism of temper brittleness of the granular bainite caused by the change of microstructure in 15CrMoVA steel during tempering is investigated. It is found that the granular bainite has obvious irreversible tempei brittleness in a wide range of brittle temperatures. The mechanism is proposed as follows. During tempering the M/A island structure decomposes and turns into plate cementite distributed heterogeneously. The undecompo-sed residue of austenite transforms into untempered martensite during tempering cooling. Under impact load, cracks initiate preferably in the plate cementite and untempered martensite. The cracks generate quasicleavage fracture while the toughness of the steel deteriorates. It is recommended that the best tempering temperature range for granular bainite structure should be selected in the temperature range where no remarkable decomposition and transformation of M/A island structure occur in order to obtain a reasonable combination of strength and toughness of the steel.