Abstract Influence of sub-zero treatments, namely: cold treatment, shallow cryogenic treatment and deep cryogenic treatment on fracture toughness of AISI D2 steel has been examined with reference to conventional heat treatment. Fracture toughness has been determined by three-point bend tests using miniature chevron-notched round bar specimens. Measurement of fracture toughness values has been supplemented by the examination of fracture surfaces, evaluation of hardness and characterization of microstructures. The obtained results infer that sub-zero treatments, in general, reduce fracture toughness of the selected steel as compared to the conventionally heat treated ones, however, the degree of reduction is lowest for deep cryogenically treated specimens and highest for shallow cryogenically treated ones. The mechanisms of initiation of fracture for all types of specimens have been identified as cracking of primary carbides and microvoid nucleation by decohesion of secondary carbides followed by cleavage, while growth of microvoids has been observed only for conventionally treated and deep cryogenically treated specimens. The variation of fracture toughness values has been explained by the microstructural characteristics of the differently treated specimens.