EARLY ESTIMATION OF BEAN YIELD LOSS IN COMPETITION WITH JIMSON WEED IS AN IMPORTANT APPROACH FOR BETTER WEED MANAGEMENT OF THIS CROP PRODUCTION. SO AN EXPERIMENT WAS CONDUCTED IN NATIONAL BEAN RESEARCH STATION, IN KHOMEIN, IN 2006. THE EXPERIMENT DESIGN WAS A RANDOMIZED COMPLETE BLOCK WITH A FACTORIAL ARRANGEMENT IN THREE REPLICATIONS. TREATMENTS WERE FOUR DIFFERENT WEED DENSITIES (4, 8, 12 AND 16 PLANT M-1) AND THREE TIMES OF WEED EMERGENCE (WITH CROP EMERGENCE, IN FIRST TRIFOLIATE LEAF STAGE AND THIRD TRIFOLIATE LEAF STAGE OF BEAN) IN COMPETITION WITH BEAN (40 PLANT M-1) AND A WEED FREE TREATMENT AS CONTROL. KROPFF & SPITTERS ONE-PARAMETER AND KROPFF & LOTS TWO-PARAMETER EMPIRICAL MODELS WERE USED TO ESTIMATE CROP YIELD LOSS BY USING OF RELATIVE LEAF AREA OF WEED AT 20, 35, 50, 65 AND 80 DAYS AFTER CROP EMERGENCE. RESULTS SHOWED THE BOTH MODELS HAD ACCEPTABLE ESTIMATION OF CROP YIELD LOSS, HOWEVER, ACCORDING TO REMAIN MEAN SQUARE, TWO-PARAMETER MODEL ESTIMATE IT BETTER. THIS MODEL ESTIMATED THE MAXIMUM YIELD LOSS OF BEAN FOR 78% AND ITS RELATIVE DAMAGE COEFFICIENT (Q) WAS 2.64 ± 0.17 WHICH SHOWED JIMSON WEED WAS THE BETTER COMPETITOR. BOTH MODELS HAD THEIR BEST ESTIMATION WHEN THEY FITTED TO THE LEAF AREA DATA AT BEAN FLOWERING STAGE (50 DAYS AFTER CROP EMERGENCE).