This paper discusses the method to estimate a driver's fatigue through steering motion. Several methods are known to estimate driver's fatigue. In this research, the chaos theory was applied to explain the change of steering wheel motion. If there is chaos in the motion, a strange trajectory called attractor can be found by applying the Takens' theory of embedding. Experiments were conducted on a driving simulator. Steering wheel angle signals were processed first by fast Fourier transform (FFT) and also by the wavelet transform to determine which is better. Then attractor was plotted to find out the chaos characteristics, which can indicate that a driver is fired of driving and he/she may feel fatigue.