In this paper, we performed an insulation diagnosis technique for oil-immersed power transformers by an acoustic detection method. Acoustic signal detection circuit was designed and fabricated to detect acoustic signal with high sensitivity. Electrode system such as needle to plane electrode was fabricated to simulate a defect of oil-immersed power transformers. We could find the position of the partial discharge (PD) source in 3-dimension with an error margin of 4 % in the experiments by calculating the position of the PD occurrence using the time difference of arrival measured by five AE sensors.