IN ORDER TO DETERMINE THE CRITICAL PERIOD OF WEED CONTROL IN CANOLA (BRASSICA NAPUS L. VAR HYOLA 401) IN NORTH KHOZESTAN CLIMATE, AN EXPERIMENT WAS CONDUCTED IN 2007-2008 ON THE BASIS OF RANDOMIZED COMPLETE BLOCK DESIGN WITH FOURTEEN TREATMENTS AND FOUR REPLICATIONS. TREATMENTS WERE CONDUCTED OF CONTROL AND INTERFERENCE OF EMERGED WEED FROM SEED BANK OF SOIL IN PHONOLOGICAL STAGES OF CANOLA SUCH AS 2 LEAVES, 4 LEAVES, 6 LEAVES, 8 LEAVES, AND EARLY OF FLOWERING, 50 PERCENT OF SILIQUE (POD) SET AND HARVEST (FOR CHECK). RESULTS OF THIS EXPERIMENT, IN ADDITION OF WEEDS DIVERSITY IN THE REGION, SHOWED A DIFFERENCE IN TIMES OF APPEARANCE AND STARTING COMPETITION. TREATMENT OF CONTROL AND INTERFERENCE OF WEED HAD SIGNIFICANT EFFECT ON ECONOMIC YIELD OF CANOLA. THE HIGHEST AND THE LOWEST GRAIN YIELD WERE OBSERVED AT CONTROL AND INTERFERENCE TREATMENTS, RESPECTIVELY. CRITICAL PERIOD OF WEED CONTROL IN CANOLA WITH 5 PERCENT YIELD LOSSES WAS ACCEPTABLE IN RANGE OF 31 TO 90 DAYS AFTER EMERGENCE (59 DAYS) MEANS IN 4 LEAVES TO EARLY OF FLOWERING STAGES, AND WITH 10 PERCENT YIELD LOSSES WAS IN RANGE OF 41 TO 68 DAYS AFTER EMERGENCE (27 DAYS) MEANS OF INITIATION OF 6 LEAVES TO 8 LEAVES.