The present invention relates to method and apparatus for improving video quality. The present invention provides a unified loop filter including: a pixel determining unit which determines the type of a pixel based on boundary strength; a similarity transforming unit which transforms a nonlinear filter into a nonlinear similarity-ordered statistics filter; and an integrating unit which integrates the nonlinear similarity-ordered statistics filter with a linear image filtering portion. The unified loop filter is applicable to filter reconstructed frames when an encoder or a decoder is processing a video signal.