Abstract: Many problems would be caused by sodium vapor which evaporated from sodium pool on the operation of sodium cooled fast reactor. Such as the sodium vapor that comes from sodium pool of the main vessel will deposit on the cool wall of annulus among the rotating plugs which will bring rotation troubles. The study on the evaporation behavior of metal sodium is the first things to solve the problems. Therefore, sodium evaporation pots and device which were designed to study the sodium evaporation velocity at different temperatures and different argon pressures were introduced in this study. In an inert gas glove box, quantified metal sodium was transferred into a sodium evaporation cups which would be put in the sodium evaporation pot, and then installed in the sodium evaporation device. After a certain time of evaporation at different temperatures and different argon pressures, residual sodium was measured again in the inert gas glove box to calculate the quantity difference that the sodium evaporation velocity was obtained. The precision of temperature control was ±1 ℃. The precision of pressures control were ±0.06 MPa, ±1.5 kPa and ±1.3 Pa while measuring the argon absolute pressure of 5 Pa, 50 kPa0.6 MPa and 1.15.1 MPa, respectively. By a large number of experiments, the sodium evaporation velocities were obtained at the temperature of 200550 ℃ and the argon absolute pressure of 5 Pa5.1 MPa(a). The results show that sodium evaporation velocity increases with the temperature while decreases with the increase of argon gas pressure. Sodium evaporation velocity increases with the reduction of distance between testing cup and cooling coil. By the fitting of experimental data, the rule was found that sodium evaporation velocity increased exponentially with the temperature at the same argon pressure which had been satisfactorily described by Langmuir formula. At the same time, sodium evaporation velocity was found to be decreased in logarithm with the increase of argon pressure at the same temperature which was related to quadrinomial logarithm of argon pressure. A fit formula was given by considering the comprehensive influence of temperature and argon pressure. And another formula was given in a simple type for the sake of convenience use. The results obtained in this study could be applied to estimate the sodium evaporation at the range of 200550 ℃ and 5 Pa5.1 MPa(a). So that they will be useful in the safety operation and maintenance of sodium cooled fast reactor.