ABSTRACT Two field experiments on sunflower were conducted during 2013 and 2014 summer seasons at EL-Serw Station, Damietta Governorate, Egypt. Each experiment included ten weed competition and weed removal treatments either at early or late times after sowing which were: - weed competition for the whole season, weed competition for 2, 4, 6 and 8 weeks from sowing, weed free for 2, 4, 6 and 8 weeks from sowing and weed free for the whole season, to determine when a natural infestation of weeds start to reduce sunflower yield and when to control without yield losses in sunflower. Dominant major weeds in experimental fields were Portulaca oleraceae, L.; Corchorus olitorius, L.; Amaranthus caudatus, L.; Echinochloa colonum, L. and Chenopodium album, L. The obtained results revealed that which weed infestation rate under sunflower field was 5.885 and 6.527 ton fresh weight / fad.,which reduced drastically seed yield of sunflower per faddan by 55.3 to 55.2% under weed competition treatment of sunflower for the whole season in 2013 and 2014 summer season, respectively, as compared with weed free for whole season treatment. The use of response curves with weed free or weed competition period showed that seed yield and oil yield of sunflower were the highest with the field free from weeds until 6 weeks after sowing , and the critical period of weed / sunflower competition was between lie  2– 6 weeks after sowing. In regression approach for sunflower seed yield, oil yield and weed free or weed competition period the polynomials (linear and quadratic) and logistic functions were tested and quadratic function was fitted to estimate the expected yields which had the high significant with the data recorded and have the highest values of R² than the other models (linear or logistic model.) for this reason it is used to estimate critical periods of weed competition with sunflower. Also, results showed that the quadratic equations were significant and had the highest R² (0.989, 0.982) and (0.989, 0.984) for weed free period, and (0.899, 0.992) and (0.919, 0.994) for weed competition durations in 2013 and 2014 summer seasons, respectively. According to these results, the critical period of weed competition to sunflower is the first six weeks period from sowing was required to be weed free showed that to maintain 95% of maximum seed and oil yield of sunflower and one week of weeds infestation can be allowed after sunflower sowing without seed and oil yield sunflower reduction. The information should be taken in consideration for sunflower growers to plan their strategies of integrated weed management for this important crop.