Center pivot systems usually use proportional method for chemigation. The uniformity coefficient of chemical applied can be calculated using the modified formula of Heermann and Hein. When chemigation starts or stops, the chemical applied by the sprinklers along the lateral is not uniform, which will result in a bad uniformity in the chemical application area. A new mathematical equation, for calculating the amount of net water applied by each sprinkler between the time interval of the first sprinkler and the last one beginning to apply chemical, is proposed in this paper. By using the developed equation, a derived result indicated that as the total number of the sprinklers along the lateral ranges from 80 to 200, the effective chemical area is about one half of the total fan-shaped irrigated area during the time interval. In order to improve the chemigation uniformity, a proper procedure of operating the center pivot is presented.