The factors causing heart rate variability (HRV) were investigated under various kinds of workloads and flight conditions. Frequency analysis of heart rate, and cross-correlation analysis with blood pressure and respiration, were performed by using the fast Fourier analyser. Consequently, the main frequency of HRV at rest consisted of 0.1 Hz and 0.25-0.3 Hz. These fluctuations disappeared under physical loads, but the value of 0.1 Hz at first increased under moderate mental load and thereafter decreased as the mental load increased. It was suggested that the behavior of the frequency 0.1 Hz under mental load was influenced partly by higher brain centers. For the actual flight, the frequency decreased under takeoff and landing, and remained as at rest during the gunnery and acrobatic training flight. It was concluded that the evaluation of workloads under flight conditions by using the analysis of the factors of HRV was very useful to improve a man-machine interface.