Abstract: A new steganalytic technique capable of reliable detection of spatial LSB steganography is proposed.First,the distribution of the difference between the current pixel and its neighborhood average is statistically modeled,and the n the variance of this distribution is defined as the image smoothness.Based on the analysis of the effects on the image smoothness brought by message embedding and LSB plane flipping, an estimation of the secret message length is derived.The algorithm can accurately estimate the amount of hidden messages and detect the existence of hidden messages embedded in the image simultaneously.It has also a distinct physical meaning,and is easy to implement,computationally efficient,and is more suitable for real-time detection.Experimental results show that the proposed algorithm is effective.