The stability of natural convection in thermally modulated inclined fluid layer is analyzed using linear instability analysis and generalized energy stability theory. A sufficient condition for the global stability of the fluid layer is obtained. The stability boundaries are found in terms of the Rayleigh number. Shooting method is used to find the stability limits numerically. Uncertain stability region is observed between the linear and the nonlinear stability boundaries. The onset of instability depends upon the frequency and the amplitude of modulation.