Based on the results of theoretical study and experimental survey on the characteristics of mix traffic flow at intersections such as traffic volume, velocity and acceleration, this paper introduces the method of determining the function area of signalized intersections in urban areas of Vietnam. Applying the proposed method to compute functional areas of signalized intersections in the cases of one-lane, two-lane (with and without a median), three-lane (with median) approach. The results of functional areas for each type of approach in different cases of traffic volume, acceleration, cycle time and left turn proportion are presented in the tables, convenient for use.