How to identify chaos is the foundation of analysis, prediction and control of nonlinear time series.A new method in this paper is put forward for calculating the largest Lyapunov exponent by utiliting the C-C method to estimate parameters in the phase space reconstruction,the ergodicity of chaos and the definition of the exponents,which remedies the defect of calculating Lyapunov exponents in some literatures.Typical examples prove the efficiency and accuracy of the new algorithm.