The theoretical analyisis of thermal instabilities of a horizontal fluid layer in plane Poiseuille flow was investigated. The unperturbed basic temperature profile was approximated, using the thermal boundary layer thickness and the wall temperature gradient. The results agreed reasonably well with the exact solutions. Applying this modified temperature profile, the conditions marking the onset of instability due to buoyant forces were obtained for large Prandtl number and Peclet number. The present theory was found to produce the result consistent with those of the extant experiments. The validity of the modified concept that thermal disturbances are confined within the effective thermal depth was confirmed.