To examine the characteristics of turbulent flow and heat transfer about the compound channel with gap, the present study has performed a numerical analysis by using large eddy simulation (LES). Our first aim was to analyze the cause of pulsating flow by investigating the turbulent flow through the unsteady analysis of compound channel. Next, to understand the correlations between gap width and heat transfer enhancement, we tried to verify the enhancement effect of heat transfer when a gap exists within the compound channel. From this numerical analysis, we could see that the lateral velocity occurs periodically around the gap and this is developed in the pulsating flow. The pulsating flow has generated a strong turbulent flow mixing within the compound channel. And the turbulent flow mixing in the pulsating flow plays an effective role of enhancing heat transfer effect by making the fluid temperature uniformly within the compound channel.