In this paper a new method for selecting the optimal kernel width for non-local means denoising is proposed.The rule of the stopping time is based on signal noise ratio(SNR).In order to correctly estimate the level of noise in non-local means denoising,a pure synthesis noise as an auxiliary is synchronously smoothed with the image.The auxiliary noise is taken as the estimation of the remaining noise in the estimated image.A new non-local means smoothing operator that can ensure the synchronous c...