The aim of the present study is to present improvements of mechanical seal performance for use under high-temperature and high-pressure conditions. Existing mechanical seals subjected to both high and low pressure show unstable leak rate and sliding reliability. Under high-temperature conditions, the temperature of seal face increases and thermal deformation of seal face takes place, leading to a decline in seal performance. We consider a new structure of the mechanical seal that cools the seal ring and seal face quickly, as well as the roughness and balance ratio of the seal to design. We derive the shape of the mechanical seal from experimental data. We also confirm that this seal exhibits the stable leak rate and sliding reliability under these conditions.