In this paper we investigate the effect of thermal fields on the morphological stability in a directional solidification system with remote flow. We undertake a linear analysis and develop an asymptotic solution for large Schmidt number. The result shows that the system is more stable when the latent heat increases and/or the ratio of conductivity of solid to that of melt gets smaller. The lead/tin alloy is taken for a realistic example and the result shows that the freezing lead/tin system is more stable when the thermal effect is taken into consideration.