Recently it has been found that some NiTi-based alloys may undergo the forward martensite transition on isothermal holding. Moreover, such isothermal transformation under stress is accompanied by variation in reversible strain. At the same time, theoretical models do not allow describing the recoverable strain variation during holding. The aim of the present study was to adjust the microstructural model earlier developed by V. Likhachev and A. Volkov for describing strain variation due to the formation of the martensite phase on holding of NiTi-based alloys under a constant stress at temperatures within the temperature range of the forward martensite transformation. To take into account the possibility for isothermal martensite formation, a new suggestion was made, according to which the isothermal kinetics might be controlled by some relaxation process, which could change the local density of point defects and led to the fulfillment of thermodynamics condition for transformation. To include this assumption into the model some modifications have been added to constitutive equations. The modified microstructural model was used to simulate the strain variation, caused by isothermal martensite formation under various stresses. The influence of holding parameters (temperature and stress) on the maximum isothermal strain was found, and a good agreement between the simulated and experimental results was obtained. It was shown that the modified microstructural model allowed predicting the holding temperature and the stress at which the maximum isothermal strain can be found.