A review is made of calculation methods for estimating the Doppler Effect in fast reactors. The minimum amount of capturing isotope U-238 is calculated that must be mixed with fissile isotope U-235 in order to ensure negative net value of temperature coefficient of reactivity for 100 KeV neutrons at a temperature of 625°K. It is found that the ratio of U-238 to U-235 nuclei must be larger than 1.9 for ensuring negative temperatures coefficient of reactivity.