We proposed a thermo-acoustic transmitter in liquid using a metal heater covered with silicone oil. Our experiments showed that the structure of silicone oil on metal heater emitted stronger ultrasound compared to the structure of water on metal heater. However, when used in other liquid, the silicone oil must be encapsulated to prevent the mixing of the two liquids. We therefore proposed the device structure of silicone oil on metal heater encapsulated by a thin film of Parylene (1 孩. The acoustic impedance gap between the silicone oil and liquid medium is small, resulting in the reduction of ultrasound reflection at the interface of the two liquids. Therefore, the thermal induction mechanism can emit short ultrasonic pulse without acoustic ringing. Our prototype device was confirmed to be able to emit short ultrasonic pulses (20 峩 over the range of 50-150 kHz. We also confirmed that, compared to the structure of metal heater put directly in water, the proposed structure enhanced the amplitude of the emitted ultrasound by 3 times.