Abstract: The reactivity temperature effect of CFBR-II reactor is analyzed with single-group perturbation theory, the calculation formulas of the reactivity temperature coefficients of CFBR-II reactor under steady-state and pulse-state are obtained, and the factors that have influence on reactivity-tempera-ture-coefficient of CFBR-II reactor are discussed. These results show that the reactivity temperature coefficients of CFBR-II under steady-state and pulse-state are negative, which values are not equal. The reactivity temperature coefficient of CFBR-II under steady-state is depended on the reactor structure and materials, the thermal exchange conditions in reactor core and between reactor and surroundings, which value is independed on the reactor power. The reactivity temperature coefficient of CFBR-II reactor under pulse-state is an ap-proximate constant during pulse, which value is depended on the reactor structure and materials.