SCADA RTU is a remote control unit equipped in substations for automatic operation of a power system. The RTU monitors and measures the operation status of remote substations to transmits the signals to upper SCADA system and receives control command signals from dispatch center to actuate corresponding apparatus. The RTU receives the standard time from GPS receiver to synchronize the times for all devices and acquires the status event of power apparatus using DIM. Without the time synchronization between GPS and RTU, stable operation of the power system and accurate analysis of the system fault are impossible due to the time error of SOE. In this paper, a test is performed to identify if the operation time of SCADA RTU DIM coincides with the standard time. And we tried to find the reason of error, to minimize the time discrepancy. Through this study, RTU operation time could be synchronized with the standard time within 10[ms] to improve the reliability of SOE data.