A Control System of automatic voltage regulator for tap-changer operation on a power transformer has been designed. This system used an AT89S51 series microcontroller as the main control unit of relays connected to the motor-drive of tap-changer. The operation principle of the microcontroller was on-of action. The microcontroller activated the relays and sent information to a computer based on conditioned signal given by a sensing unit. This signal was converted to digital signal by an analog-to-digital converter of ADC0804 series. A testing of hardware and software was conducted. The input voltage was varied up and down from nominal voltage of 110 Volt causing the rotor of motor and relay positions change accordingly. The change of voltage and relay positions could be sent on the software display using visual basic programing. The hardware and software were able to integrate and operate properly.