In general, a threshold T is used to determin whether high-frequency wavelet coefficients to be insignificant in wavelet transforme coding of singals. That is, let x be a high-frequency wavelet coefficient, if |x|≤T, then x is insignificant and set x=0. In this paper, an optimum threshold T_opt is given theoretically and T_opt minmizes the average energy of the distortion between the original signal and the reconstructed signal.