A theoretical analysis of thermal instability driven by buoyant forces under transient temperature fields is conducted in an initially quiescent, fluid-saturated, horizontal porous layer. Darcy’s law is used to explain characteristics of fluid motion and linear theory is employed. Under the principle of exchange of stabilities, the stability analysis is conducted by using propagation theory which adopts the thermal penetration depth as a characteristic length scaling factor. The critical conditon of onset of buoyancy-driven natural convection is governed by the Darcy-Rayleigh number, as expected. It is shown that the larger Darcy-Rayleigh number becomes, the earlier convective motion sets in. Based on the present critical condition to the onset of natural convection, a new heat transfer correlation is proposed. The prediction agrees well with experimental results.