Partial Discharge (PD) can be monitored by detecting the signals radiated from PD source. Ultra High Frequency (UHF) sensors have been used to capture the signals. However, external sensors are required to enhance the feasibility for applying the technique to operational transformers. In this paper, the electromagnetic waves escaped from gasket for sealing transformer tank is investigated. Finite Difference Time Domain technique (FDTD), which is widely applicable to the electromagnetic computation, is used to model wave transients. The model consists of a tank, oil insulation, a gasket and a hatch plate, which are similar to the real transformers. The wave transients at different locations are simulated. The most favourable places for positioning the sensor are discussed in accordance with the simulated results.