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In this article, fracture toughness of austenitic–martensitic functionally graded steels fabricated by electroslag remelting with crack arrester layers is investigated by experimental and analytical methods. The material contains austenite phase in addition to martensite layer. The Young’s modulus and the Poisson’s ratio have been assumed to be constant, while other mechanical properties like the yield strength and the strain hardening exponent vary exponentially along the specimen width. In analytical case, unloading compliance method is modified to calculate the critical value of J-integral for cracked three point bend specimens while standard specimens with different crack lengths are tested in experiments. The effect of crack length on the fracture toughness has been studied. It is observed that, as the crack tip goes toward a martensite layer, fracture toughness of the specimen decreases considerably. The obtained results from the proposed model are in good agreement with the experimental results.