The relationship between the critical time and evaporation parameter is discussed in terms of the kinetic model for surface segregation in the case of evaporation. An interesting phenomenon is observed that the variation of critical time obeys a Câtype law in kinetic curves with different evaporation parameters. The Câtype law of critical time is found to be caused by that of maximum evaporation flux. The existence of Câtype law is proved by the fitted kinetics curve of the Mg surface segregation of Alâ0.8 wt% Mg alloy, which confirms the model of surface segregation in the case of evaporation to be reliable.