Nondestructive testing technique using T-ray (terahertz ray) has received more interest as a new capability tool in engineering application. So, T-ray time-domain spectroscopy was used for inspecting and evaluating physical property and defect characteristics in FRP composite materials for a use of wind energy. First of all, a general refraction mode and transmission mode was utilized for calculating a refractive index (n) based on the well-machined sample. In order to obtain the terahertz ray refractive index, a refractive mode and a transmission mode technique was defined for solving a refractive coefficient (n), with which refractive index of GFRP composites, balsa and epoxy for a use of wind turbine blades could be obtained. A method of wind turbine blade composites having one and two saw-cuts delaminations and some angles/ directions was suggested based on the E-field vector. The time of flight in T-ray using evaluating a T-ray transmission mode agreed with that using T-ray time-domain spectroscopy with the wind turbine blade. Also, in CFRP composites, T-ray propagation is limited because the carbon fibers are blocking the T-ray. Therefore, we have analyzed difference between E-field vector in T-ray and the carbon fiber direction. Also, it was found that a T-ray time domain data in thick GFRP composite laminates seemed to show peaks with a regular spacing (${Delta}t$) which was obvious with the close relation between TOF and FFT.