[en] The relationship between the threshold values of ion-temperature-gradient instabilities and the temperature parameters of plasmas is investigated analytically in slab and toroidal geometries separately. It is found that the threshold values increase rapidly when the ion temperature becomes much higher than the electron temperature. The change of the threshold vaues with respect to the ion temperature is quite similar for both geometric models. This finding is consistent with PLT observations. Furthermore, the analytical results also agree with those of the numerical calculations