An on-line ultrasonic detector was developed to continuously monitor the ultrasonic signal due to partial discharge in transformer in service. The on-line ultrasonic detector has a band-pass filter designed to measure only the frequencies from 50 to 300[㎑] of ultrasonic signal, to remove electrical and mechanical noises from outside of the transformer, and tlle ultrasonic sensor contains a pre-amplifier with 60[dB] gain. The ultrasonic signal discrimination algorithm which discriminates the ultrasonic signal duration was developed to remove the ultrasonic signal due to OLTC operation having similar characteristics to those due to partial discharge. The reliability of the on-line ultrasonic detector developed in this study was convinced of measurement the ultrasonic signals from the model. transformer in laboratory and transformer in service