In this paper, thermal stability of unsteady viscous fluid flow in a cubical cavity rotating about its vertical axis z is numerically studied. In this model, it is heated from below, where the horizontal walls are assumed to be maintained at two different temperatures and the vertical walls are taken perfectly insulated. The thermal stability has been investigated for moderate values of the Rayleigh number Ra and wave number, Taylor number and wave number and also for critical Ra at three different cases, (a) Both the bounding surfaces are free, (b) Both the bounding surfaces are rigid, (c) Surface  is rigid,  is free.