The thermal instability of plane Poiseuille flow with sufficiently small Prandtl numbers in the thermal entrance region heated isothermally from below, was investigated by the propagation theory. Also the experiment with water（Pr=6.8） in this system was conducted for flow visualization, which showed that the steady longitudinal vortex roll being periodic in the spanwise direction was induced by natural convection. As a result of stability analysis of such a shape of natural convection, it can be suggested that the flow field became to be more stable with decreasing Prandtl numbers. The approximated relation considering the effect of Prandtl numbers on the critical conditions in the thermal entrance region was represented theoretically by
Ra_c = 200（1+0.283Pr^-1）x_c^-1
This relation can be applied to the region of x_c≤0.01 with Pe≥100 and predicted the critical Rayleigh number in good agreement with existing experimental results.