This paper deals with the study on partial discharge(PD) characteristics of transformer and method of UHF sensor design for transformer PD measurement. The experiment was fabricated according to IEC-60270 and IEC-62478. The analysis of PD characteristic and spectrum of various transformer defect was performed. From the result, it was deduced that PD signal of each defect has unique frequency band, and it was also found that the improvement of sensitivity of UHF sensor which is optimized for transformer could be achieved by tuning reflection loss of sensor by forming resonance point at specific frequency.