For efficient prefiltering prior to video encoding, exact noise strength of an input video sequence should be found, but it is actually a very difficult process. This paper presents an accurate noise estimation scheme using multiresolution motion estimation in a video encoder. Firstly, if the multiresolution motion estimator finds minimum block-matching errors at the finest resolution and the middle resolution, the differences between the two block-matching errors are computed and averaged for all the macroblocks (MBs) in an input frame. Based on a property that the average difference of each frame is linearly proportional to its noisiness, the noise strength of the input frame is precisely estimated. Experimental results show that the proposed noise estimation is very accurate.