The Volkov and Evard microstructural model describing the functional properties of TiNi shape memory alloys was modified to simulate plastic strain variation during thermal cycling of the TiNi alloy under constant stress through the temperature range of the martensitic transformation. It was found that a good agreement between the experimental and simulated data was observed if the law for the variation in the critical force for plastic slip have taken into account the non-linear dependence between the variation in critical force and the decrease in this value in previous thermal cycles due to relaxation. It was shown that using a modified Volkov-Evard model allowed us to simulate plastic strain variation during thermal cycling under constant stress through a different temperature range of the forward martensitic transformation.