The parametric instability of an infnite horizontal fluid layer excited by time periodic
oscillation of the wall temperatures, has been investigated numerically, using the Floquet
analysis. It has been found that the modulation of the temperature gradient across the
fluid layer affects the onset of the Rayleigh-Benard convection. The instability zones in the
parametric space of the dimensionless wave number of disturbance and the amplitude of
modulation, have been obtained. The instability response has been found to be harmonic or
subharmonic with respect to the frequency of modulation, depending upon the modulation
parameters