Main purpose in this study is to minimize the active power losses of the distribution systems as an optimization problem. To achieve this, the tap position of the ULTC (Under Load Tap Changer) is used as the system variable. As the processual flow of the method, the tap setting of the ULTC in the IEEE 13 Bus test system, was obtained by power flow with OpenDSS software according to the predetermined load condition for 24 hours, on the first step. The step value of the transformer's output voltage of the transformer is adjusted to the desired level (usually 1 per-unit (pu) in the middle point of the feeder), in OpenDSS software. In the next step, as the main objective, an implementation is designed in MATLAB to compute the transformer tap values, which will minimize the active power losses in distribution systems by using genetic algorithms which is a heuristic method. As a constraint, the voltage level in the system is kept between 0.95 - 1.05 pu. Simulation studies have shown, loss minimization provides more effectiveness and better energy efficiency by using the step values obtained by genetic algorithms.