Two field experiments were conducted on maize during 2014 and 2015 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 maize yield and when to control without yield losses in maize. The most appropriate study of the critical period of weed competition is on the weed community as a whole than single weed specie. Dominant major weeds in experimental fields were Portulaca oleraceae, L.; Corchorus olitorius, L.; Amaranthus caudatus, L.; Echinochloa crus-gali, L. and Echinochloa colonum, L. The obtained results revealed that which weed infestation rate under maize field was 5.834 and 6.043-ton fresh weight / fed.,which reduced drastically grain yield of maize /fed by 56.8 to 57.2 % under weed competition treatment of maize for the whole season in 2014 and 2015 summer seasons, respectively as compared with weed free for whole season treatment. The use of response curves with weed free or weed competition periods showed that grain yield of maize / fed were the highest with the field free from weeds until seven weeks after sowing and the critical period of weed / maize competition was between 3 to 7 weeks after sowing. In regression approach for maize grain 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 predict yield losses due to weed competition periods with maize. Also, results showed that the quadratic equations were significant and had the highest R² (0.990 and 0.985) for weed free period, and (0.994 and 0.993) for weed competition durations in 2014 and 2015 summer seasons, respectively. The important stage of weed competition to maize is the 1-9 weeks period from sowing was required to be weed free showed that to maintain 95% of maximum grain yield of maize and one week of weeds infestation can be allowed after maize sowing without grain yield maize reduction. This information should be taken in consideration for maize growers to plan their strategies of integrated weed management for this important crop. It could be concluded that weeds should be removed within the period of first week and nine weeks in maize crop to pass up 5 and 10% grain yield reduction, respectively.