This paper deals with developing the highprecision liquid-temperature regulator to minimize thermal deformation of ultraprecision machine tool. The liquid-temperature control system was constructed based on theoretical analysis and the prototype of highprecision liquid-temperature regulator was experimentally tested for temperature accuracy under various environmental conditions. The experimental results confirmed that the temperature accuracy was ±0.0011 to 0.0012°C over a measurement period of 36 hours, during which the room-temperature fluctuated by ±0.1°C. Furthermore, when the room-temperature fluctuation was restricted to ±0.04°C, the temperature accuracy improved to be ±0.00055°C for over 36 hours.