Abstract: Bainite, ferrite and a large amount of stably retained austenite were obtained through austempering immediately after intercritical annealing of Si-free TRIP steel. The behaviour of TRIP (transformation-induced plasticity) effect was discussed through observing microstructure, analyzing fracture morphology and comparing it with the mechanical properties of high-Si TRIP steels. The results showed that Si-free TRIP steel has the excellent mechanical properties due to strain-induced martensite transformation which forms TRIP of retained austenite when it is strained, and its fracture morphology presents toughness characteristics. The tensile strength, total elongation and strength-ductility balance property of such Si-free TRIP steel all reach their maximum values (754 MPa, 36% and 27144 MPa%, respectively) when it is heated at 790°C and then held at 400° for 5 min.