This thesis presents the factors related to reactive power for power loss reduction by calculation the optimal adjustment of devices. For constant load, average adjustment when be used with daily load, selected of devices for adjustment when load change, order of control adjustment to consider of voltage profile, increase the control variables margin to used when system have a problem. Algorithm for optimal power flow to calculate optimal control variables is based on interior point method. The proposed methods has been tested with the standard IEEE test systems. The obtained results demonstrate that the power loss is reduced when control variables have been in optimal adjustment. The variables margin can be increased. Equations that presented can find the optimal location of device to reduced power loss. To used more than one level of load to find optimal variables can be reduced power loss. The control devices can be selected to have more effect of reduction power loss.