Renewable based electricity, generated from wind turbines is considered as a clean alternative energy resource. In a wind power plant, it is of vital importance to avoid resonance of the blade when a frequency of an exciting force exists, especially when it coincides with one of the natural frequencies of the system. The present study deals with the vibrational behaviour of a typical wind turbine blade, NACA 4415. Once the blade is under static load, its natural frequencies are analysed. Two energy methods of Rayleigh-Ritz and optimized Rayleigh-Ritz are employed to carry out the analyses. For the applied loads, the natural frequencies have also been specified. In case of bending is applied in vertical and horizontal directions has been investigated. There is a good agreement between the results of both methods. The results show that for the given blade profile, rotational speeds corresponding to the specific dimensional natural frequency should be avoided during the blade operation to maximize the output power of the wind turbine and extend its lifespan.