An investigation and analysis of failure accidents of Wind-lens turbine blades was presented. Due to the failure of the electric loading device, Wind-lens turbine was running under non-loading conditions for some minutes. Then, the wind velocity U around Wind-lens turbine reached U_<max> =19m/s and the tip velocity V_<tip> of blades reached V_<tip> 152m/s. Fatigue crack was easily initiated at the corner of fillets of joints connecting blades to hub where stress concentration was caused. The bolts were subjected to a large load by application of vibrational stress and torsional moment. The clearance between the head of bolts and hub was expanded. To investigate the fracture surface, the beach marks and striations characteristic of fatigue fracture were obviously observed. The number of cycles from crack initiation to final failure was calculated on the basis of fracture mechanics by observing the fatigue striation in the vicinity of fracture origin. Vibrational service stress amplitudes applied at the joint of blades and hub were much larger than the inherent fatigue strength of blade's material, ABS resin. In order to prevent such a failure accident of a small wind turbine generator, the recommended shapes of blades and hub were proposed.