Abstract: The major dispute between Mack's and Wazzan's results of the linear instability for flat plate boundary layer lies in the effect of free stream Mach number on the viscous instability. In the paper, taking influence of the air thermodynamic properties into account a temporal eigenvalue problem of an adiabatic flat plate is calculated by a collocation technique. The numerical results indicate that the viscosity is only stablizing for the first mode and the second viscous mode is always stable at M=3. It is co...