As the technology of wind turbine generators (WTG) has been largely developed, so we much depend on their turbine makers in designing. However the environmental conditions in Japan are significantly different from those where this technology originated; for example the magnitude of variability of sea conditions. Therefore we studied for the combinations of stresses due to wave and wind using time domain structural analyses subjected to fluctuating wind and wave loadings. In the study, the maximum stresses due to wave alone, ones due to wind alone and those due to both wave and wind were determined with the NOWPHASE data measured at off Miyazaki during typhoon number 18 in 1999. Furthermore we conducted Rayleigh forecasting of maximum stresses using the means stresses, and their standard deviations obtained from simulated stress histories. Making clear the relation between stresses due to wind and wave, this paper concluded that the maximum stresses may be simply the sum of both maximum stresses.