Temperature fluctuation from incomplete fluid mixing can induce fatigue cracks on structures of nuclear components, which should be prevented. For rational analysis of this phenomenon, the authors have developed a frequency transfer function that translates fluid temperature fluctuation to stress intensity factors. This function is formulated by a product of the effective heat transfer and the stress intensity factor functions, and enables us to quickly calculate stresses intensity factors induced by fluid temperature fluctuations. Furthermore, it can evaluate sensitivities of stress intensity factors to frequencies of temperature fluctuation, Biot number and constraint conditions of structures. Applicability of this function was verified through comparison with stress intensity factors calculated by the finite element method.