Studies of the effects of hydrogen (H) isotopes are an important issue on achievement of high performance in the next Deuteriun-Tritium (DT) and Tritium-Tritium (TT) high beta operations at JET with ITER-like Wall. Experiments with mixed H-D plasmas have been recently performed at JET to understand the dependence onthe confinement on these isotopes varying the gas, beam and pellet fuelling. Since strong dependence on confinement has been found in mixed plasmas close to pure values of H or D and nodependence for other values of the isotope mixture, while opposite behavior is expected for any DT composition, the presence in these scenarios of magnetohydrodynamic instabilities could still play a key role in the confinement loss. Particularly, the Neoclassical Tearing Modes (NTMs) are responsible of a decrease of performances leading in some cases to disruptions. The goal of this work is to predict the effects of different isotope mixtures on NTMs onset and dynamics for the plasma stability, depending the modes appearance on the isotope ion mass mi . The NTM modelling of the island width evolutionis performed using in the NTM module,integrated in the European Transport Simulator (ETS),a generalized Rutherford equation where the bootstrap and the ionpolarization terms contain the mi dependence through the ion collision frequency and the ion Larmor radius. Dependence of sawtooth (ST) period on the isotopes is investigated as well, because NTMs can be triggered after a crash of a sawtooth period, occurring when the magnetic shear becomes larger than a critical one (s1,cr). TheST periods in H are smaller than in D, because s1,cr(H) < s1,cr(D) , as observed in isotope identity experiments. As the mode onset is usually observed after a crash of a long ST period, the NTMs destabilization should be more favourable in D plasmas than in H and more in T than in D leading to more confinement degradation. This isotope dependenceof the periodof sawteeth is investigated.