In this paper, the critical pressure Pcrit and impeding pressure Pirnpe of the elemental evaporation were defined and studied based on the calculation of the relationship between the evaporation loss rate Nm and the chamber pressure p during melting of NiAl alloys. When the chamber pressure is lower than Perit or higher than Pimpe, Nm tends to be the maximum or minimum value and remains almost unchanged. However, declines sharply with the increase of the chamber pressure when Pcrit＜P＜Pimpe. A method has been put forward to calculate the Pcrit and Pimpe of Al evaporation in a Ni-XAl (X=25～50 at. pct) melt. The calculation result shows that the pcrit or Pimpe is a second-order function of the molar percentage of Al and the melting temperature.