This paper propose a method that GOP structure based on the picture complexity change realtime adaptive without pre-analysis or time delay. Proposed algorithm calculates the complexity of pictures at first, and the ratio of the complexity( X$sub$p/ /X$sub$i/) between P picture and I picture is calculated. The suitable M value for the three picture select by comparing with predetermined threshold. Used bit and vbv_delay the value of GOP is calculated according to selected M. Experimental results show that the prediction error is reduce than the fixed GOP structure. Since the complexity distribution of the sequence is different, applied limits of threshold value is changed, also.