After JPEG double compressed

the DCT statistic of JPEG image will be changed

which is the focus in this paper.The distributions of DCT coefficients are changed with the double compression factors: Q_1 and Q_2.The difference of the distributions is analyzed in this paper.A new method-dithering pattern is proposed to detect double JPEG compression.Meanwhile

the necessary and sufficient conditions about dithering pattern are obtained.Experimental results show that this method is effective.