Two field experiments were carried out at the Experimental Farm of the National Research Centre, at Shalakan, Kalubia Governorate, Egypt, to determine the critical period of weed competition in maize fields during 2000 and 2001 in summer seasons. The main results could be summarized as follows: -          The first 12-24 days after sowing (DAS) were the most critical period for weed removal in maize fields. -          Weed removal at 12, 24 and 36 DAS decreased the number of total weeds to 29.14, 36.57 and 44.00% in season 2000 and 27.17, 35.33 and 42.39% in season 2001, respectively and reduced fresh weight to 40.95, 44.49 and 50.23 % in season 2000 and 39.89, 45.93 and 50.28 in season 2001 and decreased dry weight to 40.10, 43.40 and 49.15% in season 2000 and 39.23, 44.98 and 49.31% in season 2001 respectively, when compared to unweeded check. -          Weed free until harvest and weed removal at 12, 24 and 36 DAS improved the growth of maize plants and produced the longest ears, the highest number of grains/row, the highest weight of ear grain, weight of 100-grain and the highest grain yield per fed as compared to unweeded check. -          Shelling percentage was not significantly affected by all weed removal period in both seasons. -          When weeds left to grow for 48, 60, 72, 84, 96 and 108 DAS, the grain yield per fed was reduced by 28.82, 33.30, 35.70, 41.19, 44.36 and 51.60% in season 2000 and 30.00, 33.12, 36.09, 39.37, 43.09 and 49.08% in season 2001 respectively, when compared with the weed free treatment. The highest percentage of protein and oil in both seasons was obtained when weed free until harvest and weed removal at 12, 24 and 36 DAS. On the other hand, further delay in weed removal for later stages reduced grain protein and oil percentage, while the least protein and oil percentage was recorded at unweeded check through the two seasons. Weed removal at 12-24 days after sowing was adequate for getting optimum yield in maize.