An experimental investigation has been carried out on the turbulent heat transfer in a separated and reattached flow around a downward facing step. Main objective of the present study is to clarify effects of the step aspect ratio on turbulent heat transfer in the separated, reattached and redeveloping flow field. Reynolds number tested was 26000 and the aspect ratio was varied systematically from 10.0 to 3.0. Heat transfer characteristics are clarified along with the turbulent velocity and temperature fields. The large scale vortex structure shed from the reattachment region and its correlation with the heat transfer behavior are investigated using the conditional sampling technique.