The realization of forward martensitic transformation under isothermal conditions in NiTi-based alloys has been well studied experimentally, along with the kinetics of this process. However, existing models do not allow the isothermal martensite volume fraction FM to be estimated, hence the influence of holding temperature or time on the FM value could not be calculated. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory is normally used to describe isothermal kinetics, but it has not been applied for martensite transformation, which occurs in shape-memory alloys during holding at a constant temperature. Thus, the aim of the present study is to adapt the JMAK theory and use to estimate FM variation with time during holding of Ti40.7Hf9.5Ni44.8Cu5 alloy at different temperatures. The JMAK equation allows the variation of the isothermal martensite volume fraction with time during holding at constant temperatures to be approximated. It was applied to estimate experimental Phi(M)(t) curves in Ti40.7Hf9.5Ni44.8Cu5 alloy and a good approximation was established. The dependencies of JMAK-like equation parameters on the holding temperature were also found and approximated. Thus, the expression for the dependence of the isothermal martensite volume fraction on holding temperature and time was found and calculation of the FM(t) curves was carried out. The simulated and experimental data for the Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy were shown to be in good agreement.