A fuzzy logic based One-at-a-Time sensitivity study method of nonlinear systems is proposed in this paper. The advantage of the method is that it can be used for a wider parameter examination range. With the use of fuzzy logic a smooth evaluation of sensitivity study could be achieved and the degree of sensitivity of parameters could be easily compared. The method was tested with 2 case studies: The sensitivity study of a nonlinear Duffing-type vibration system and a vehicle simulation using an open-source physics simulator. With the simulation results it was shown that the method can be used effectively for sensitivity study of nonlinear systems in an extended examination range.