FIELD EXPERIMENT WAS CONDUCTED TO EVALUATE THE COMPETITION BETWEEN TALL AND DWARF SOYBEAN CULTIVARS AND REDROOT PIGWEED IN THE FIELD OF AGRICULTHURE AND NATURAL RESEURCES, TEHTAN UNIVERSITY, KARAJ, (IRAN). THE EXPERIMENTAL DESIGN WAS A FACTORIAL ARANGMENT OF TREATMENTS BASED ON COMPELETELY RANDOMIZED BLOCK WITH THREE REPLICATIONS. TWO SOYBEAN CULTIVARS INCLUDING CLARK (TALL) AND INTERPRICE (DWARF) WERE PLANTED IN COMPETITION REDROOT PIGWEED AT (0, 2, 4, 8 PLANT/M2). THE RESULTS SHOWED THAT INCREASING THE DENSITY OF REDROOT PIGWEED REDUCED THE GRAIN YIELD OF BOTH CULTIVARS. WING TO THE GREATER HEIGHT AND THAS MORE LIGHT INTERCEPTION, CULTIVAR CLARK PRODUCED HIGHER YIELD THAN THE DWARF CULTIVAR INTERPRICE. YIELD REDUCTIONS FROM THE REDROOT PIGWEED DENCITY OF 8 PLANT M-2 WERE 45% AND 29% IN IINTERPRICE AND CLARK, RESPECTIVELY.