Among the components used in a wind turbine the blade is the most damageable component for fatigue caused by atmospheric turbulence. Therefor it is necessary to understand the stresses which are occurring on the operating wind turbine blade. We installed a prototype 500 kW wind turbine developed by the NEDO (New Energy and industrial technology Development Organization) in Tappi Wind Park where is very complex terrain, to study its performance, reliability and durability. Some measurements and data analysis of the blade sh-ess have been done. In this paper, some results of the stress measurement are presented.