LEAVING CROP RESIDUES ON THE SOIL SURFACE MAY SIGNIFICANTLY AFFECT THE BEHAVIOR AND ACTIVITY OF HERBICIDES. IN ORDER TO STUDY THE EFFECT OF WHEAT RESIDUE AND DIFFERENT RATES OF TRIFLURALIN (%48 EC) HERBICIDE ON WEED COMPOSITION OF SUNFLOWER (HELIANTHUS ANNUUS). A FIELD EXPERIMENT WAS CONDUCTED AT THE RESEARCH STATION OF AGRICULTURAL FACULTY; THE UNIVERSITY OF BIRJAND DURING 2008.EXPERIMENTAL DESIGN WAS RANDOMIZED COMPLETE BLOCK DESIGN AS A FACTORIAL EXPERIMENT. TREATMENTS WERE RATES OF WHEAT RESIDUE SURFACE (0, 1250, 2500 AND 3750) KG HA-1 AND RATES OF TRIFLURALIN (%48 EC) HERBICIDE (720, 1200 AND 1680) G AI HA-1WITH AS CONTROL (WITHOUT HERBICIDE USE). RESULT SHOWED WHEN USING WHEAT RESIDUE INCREASED TO 1250 KG HA-1 HERBICIDE EFFICACY WAS HIGHER THAN THE CONTROL. BUT USING 3750 KG HA-1 WHEAT RESIDUE DECREASED HERBICIDE EFFICACY.