In this article, two key pressures, the critical pressure P crit and the impeding pressure P impe of elemental evaporation, are defined and studied based on the calculation relationship between the evaporation loss rate N m and the chamber pressure P during melting of titanium-aluminum alloys. When the chamber pressure is below P crit or above P impe , N m tends to the maximum or the minimum value and remains almost unchanged. However, if P crit Ͻ P Ͻ P impe , with the increase of the chamber pressure, N m declines sharply. A method has been put forward to calculate P crit and P impe of Al evaporation in a Ti-XAl (at pct) (X ϭ 25 to 50) melt. The calculation result shows that P crit or P impe is a secondorder function of the molar percentage of Al and the melt temperature.