Abstract: Based on chaos theory and the nonlinear dynamical systems, the electrode displacement signal was studied in the spot welding process.Four quality states of spot welding were studied and the largest Lyapunov exponents are calculated.These results show that the greatest Lyapunov exponents are positive number in the four different displacement signals. According to the theory of phase space reconstruction and Lyapunov exponent, there are some chaotic characteristics in the displacement signal of spot welding and relationships between spot welding quality and Lyapunov exponent.This research provides a new method to measure and forecasting the spot welding quality.