TO EVALUATE THE EFFECT OF DIFFERENT SOWING DATES, HERBICIDE AND RICE GENOTYPES ON BIOMASS AND DENSITY OF WEEDS IN DIRECT SEEDING RICE, AN EXPERIMENT WAS CONDUCTED IN 2008 AT AMOL, USING SPLIT- SPLIT PLOT BASED ON RANDOMIZED COMPLETE BLOCK DESIGN WITH THREE REPLICATIONS. THREE DIFFERENT SOWING DATES (APRIL 24, MAY 4 AND MAY 14), THREE TYPES OF HERBICIDES (SUNRICE, TOPSTAR AND COMBINATION OF SUNRICE+TOPSTAR) AND SIX RICE GENOTYPES (TAROM MAHALI, TAROM HASHEMI, FAJR, BAHAR1 HYBRID, LINE 2 AND LINE 18) WERE AS MAIN PLOT, SUB PLOT AND SUB-SUB PLOT, RESPECTIVELY. RESULTS INDICATED THAT ALL FACTORS, EXCEPT HERBICIDE AND GENOTYPE FOR WEED NUMBER PER M2 AND HERBICIDE FOR WEED DRY WEIGHT, HAD SIGNIFICANT EFFECT ON ALL STUDIED TRAITS. THE MINIMUM WEED DRY WEIGHT WAS RECORDED WHEN MIXED HERBICIDE AT APRIL 24 OR MAY 14 AND SUNRICE AT MAY 4 WERE APPLIED. THE LOWEST WEED DENSITY WAS OBTAINED IN TOP STAR HERBICIDE AND SECOND PLANTING DATES. IN TERMS OF WEED DENSITY AND BIOMASS EARLIER PLANTING DATE AND APPLICATION OF SUNRICE OR MIXED TREATMENT HAD BETTER PERFORMANCE IN ALL GENOTYPES EXCEPT BAHAR 1 HYBRID.