Summary: Past experimental studies have revealed that TRIP steel possesses favorable fracture toughness because of strain-induced martensitic transformation during large plastic deformation. However, problems associated with the mechanism of high fracture toughness are still unclear. The prediction and control of the deformation and transformation behaviors of TRIP steel near the crack tip are indispensable for obtaining the required fracture toughness. Here, the deformation behaviors of CT specimens of TRIP steel are simulated by FEM under mode I loading at various temperatures for the cases of stationary cracks and stable crack extensions. In the case of a stable crack extension, the nodal release technique and concept of pull-back force are introduced. The fracture toughnesses for TRIP steel and an austenitic material without martensitic transformation are also calculated by path integrals such as \(J\) and \(T^{\ast}\) integrals. The mechanism of high fracture toughness due to strain-induced martensitic transformation for the case of stationary crack is discussed by comparing the computational results for TRIP steel with those for austenitic material. Then, the mechanism of strain-induced martensitic transformation toughening for TRIP steel due to crack extension is investigated.