UPS(Uninterruptible Power Supply) have been used to improve power source quality as well as to protect critical loads such as computer, industrial system, medical equipment, etc, by providing clean and uninterrupted power under power supply disturbance or interruptions. But UPS is bulky because batteries. So, a UPS size issue has received the attention in the power electronics industry. This paper proposed an implementation of the computer power supply with a bulit-in UPS controlled by the one-chip microprocessor to minimize UPS size. The experimental results verified the proposed SMPS with a bulit-in UPS for computer.