There are three well-known methods for computing lower critical Mach number of airfoils. The method of Prandtl-Glauert is an approximate method as it is based on a linearized approach. Two other methods based on a non-linearized approach are found in literature. They are methods of Karman-Tsein and Christianovitch. The latter is more accurate than the former, but it is of less accuracy. This paper presents a simple method of determining lower crit-ical Mach number of airfoils. The present method is as accurate as the method of Christianovitch. In the mean time, it is con-venient to computational aerodynamics.