Four field experiments were conducted at Horticulture Research Station, El-Kanater El-Khiria, Kalubia governorate in two successive winter seasons 2016/17 and 2017/18. First, two of themwere carried out to determine the period required for weed free maintenance after carrot sowingto produce the maximum yield and the long of weed competition period which can be allowed without reduction in the yield.Each experiment contained twelve treatmentsi.e six weed free periods at 2, 4, 6, 8 and 10 weeks after sowing and for the whole season and six treatments of weed competition period at 2, 4, 6,8 and 10 weeksfrom sowing and for the whole season.Second,of themtwo field experiments were carried out to evaluate the efficacy of some promising pre emergence soil acting herbicides on weeds and root yield of carrot. Each experiment including seven weed control treatments namely hand weeding at twice,metribuzin at the rate of 100 g/fed., linuron at the rate of500cm³/fed., pendimethalin at rate 1.7 liter /fed., butralin at rate.2.5 liter/fed., as post sowing  pre- emergence,prometryn the rate of  1.25 liter./fed.as post  sowingandwell as the unweeded treatment.The main findings showed that weed infestation for whole seasonreduced yield of carrot per fed. by 47.1 and 47.0% in both seasons, respectively, than weed free for whole season treatment. Mathematized models which given the relationship between weed infestation periods and carrot yield show that quadratic equation was the best model to predict yield losses due to weed competition is showing that thecritical periods length of weed competition nine weeks period from sowing which is required to be eliminates to obtain the maximum carrot yield of carrot without reduction.Weed free for all season gave the best value for all characters per faddan.In the second study, results show that the use of linuron at the rate of 500 cm3/fad gave the highest controlling percentages of the annual broad leaf and grassy weeds through this period with the highest values of carrot yield and can be advised for weed control in mentioned critical period of carrot. At harvest there is no herbicides residues were detected intheedible carrot yield.