The article introduces the research result about the impact of the left-turn vehicles to the capacity of the signalized intersections in our urban traffic condition. Based on the real surveys at many intersections in Danang and Hanoi City, after that testing by the Traffic Brain software (TB) for the popular cases; from there, author builds relationships by function between the proportion of the left-turn vehicles to the capacity of the approach. This results allow to determine the biggest real capacity of the approachs, then makes the basis for designing the geometry of intersection, calculating optimal controlling cycle and separating phasing reasonably.