The accuracy of the numerical simulation of casting solidification largely depends on the selection of appropriate thermal boundary conditions. The estimation of heat flux at the metal-mold interface becomes difficult due to the formation of spatially and temporally varying air gap in gravity die-casting. However, the spatial variation in the formation of air gap is often neglected in most of the previous research work. In this paper, an experimental setup that involved mold filling was devised In order to study the spatial variation of air gap and its effect on the heat flux at the metal-mold interface. A Serial- IHCP (inverse heat conduction problem) algorithm was used to estimate the multiple heat flux transients along the metal-mold interface. The estimated heat fluxes at the metal-mold interface have considerable variation during the initial stages of solidification. Further, the analysis indicates that the nonconformal contact between metal and mold begins at the bottom. However, the formation of clearance gap at the metalmold interface follows a reverse trend as it starts from the top of the vertical mold wall and progresses towards the bottom. Copyright © 2007 by ASME.