In the development of wind turbine technologies, nowadays, doubly-fed induction generators (DFIGs) are becoming the dominant type due to their advantages of variable speed operation, four-quadrant active and reactive power capabilities, independent control of their active and reactive output powers, high energy efficiency, and low size converters. Due to the popularity and wide usage of DFIG systems for wind energy generation, nowadays, control systems suitable for this application have been extensively investigated as well. The most popular and practical control scheme of DFIGs is vector control based on proportional-integral (PI) controllers. Due to the simplicity and reliability, PI controllers have been popular in the engineering field. However, adjustment of these parameters is a very important and considerable problem. Thereby, in this thesis, optimized adjustment of PI controller parameters of wind turbine based on DFIGs is focused. According to the flux continuity principle, once a doubly-fed induction generator (DFIG) is subjected to a disturbance, for example by a change in the wind speed, the stator flux cannot change instantly. This fact causes the appearance of DC component in the stator flux. Under this condition, rotor back-EMF voltages reflect the effects of stator dynamics on rotor current dynamics, and have an important role on the oscillations of the rotor current. To damp these oscillations and to improve system dynamic performance, focusing only on small signal analysis may not be efficient. So in this thesis, a new method is proposed based on combination of small signal theory and discrete Fourier transform (DFT) technique to quantify damping characteristic of rotor current during occurrence of bigger disturbances. Base on this method, frequency spectrum of a signal which is available by the help of DFT, gives us the information of damping of each frequency component. So, in this thesis to improve the dynamic performance of DFIG system by using the multi optimization technique based on above mentioned method, optimized adjustment of PI controller parameters are achieved. Optimization problem is solved using NSGA-II algorithm which is a version of genetic algorithm and designed for multi objective optimization problems. Considering small signal stability, the main purpose of the control system in the present thesis is to increase the system damping ratio as well as to guarantee enough stability margin. Eigenvalue analysis and time-domain simulations have been presented to demonstrate that the proposed optimization method yields better control performance in comparison with one designed using mere eigenvalue relocation. In the following, in order to find which controller parameters are the most effective ones, trajectory sensitivity analysis is used. It is shown that four control parameters have the most effect on the dynamic performance of the system. Thereby, optimization adjustment algorithm with these four controller parameters is corrected. Introduction of the bifurcation theory and the use of this theory on the DFIG system are another research subjects that is followed in this thesis. The influence of control and non-control parameters in the occurrence of bifurcations has been investigated. Also, stable domain of effective controller parameters in one, two, three and four-parametric form is extracted by the help of this theory. This information could be useful when proposed optimization parameters adjustment algorithm is developed. The proposed method containing the algorithm and determination of stable domain of controller parameters based on bifurcation theory, determination of more effective controller parameters based on trajectory sensitivity analysis and determination of optimized controller parameters based on multi objective optimization using small signal analysis and DFT analysis of rotor current time wave form is presented for the first time to improve the dynamic performance of DFIG system. Key Words DFIG, optimized control, DFT, NSGA-II, trajectory sensitivity analysis, bifurcation theory