The investigation of complex material behaviour of steel like transformation-induced plasticity (TRIP) is a large field of current research. The simulation of the material behaviour of work-pieces in complex situations requires a knowledge as deep as possible about phenomena like TRIP. In addition, there are effects in the case of non-constant stress which cannot be explained by the widely used Leblond model. Therefore, we consider a TRIP model taking into account backstress due to TRIP itself. Assuming a linear dependence of this backstress from the TRIP strain we derive formulas for calculation of the new TRIP parameters. Based on experimental data we calculate these parameters.