Abstract: Mechanical seals are widely used in the primary circuits in all kinds of nuclear power plants. In the nuclear power plant, whether a long-period safe and reliable operation can be achieved is largely determined by the quality of the mechanical seal. So mechanical seal used in the primary circuit is regarded as one of the most important parts in the nuclear power plant. In this paper, mechanical seals applied in several typical reactor types were introduced, where water, helium and liquid metal were applied as coolants separately. They covered the range from the second-generation to the fourth-generation nuclear technology. For a start, mechanical seals used in reactor coolant pumps of the pressurized water reactors were illustrated. Their structure, operation principal and recent research status both in American-style, most of which were based on the hydrostatic principal, and European-style, most of which were based on the hydrodynamic principal, were described in detail separately. Then, introduction about the structure, use requirements and specific design of the dry gas seal used in helium turbine or helium fan in the High-temperature gas-cooled reactor are presented. Finally, two kinds of mechanical seals which were used in the sodium pumps of the sodium-cooled fast reactors were introduced. One was inert gas buffered, oil lubricated, three-stage series seal, and the other was the spiral groove mechanical seal directly lubricated by the liquid metal.