The aim of this study is to clarify the characteristics of the heat transfer enhancement in the downstream region of a backward-facing step flow using a small amount of mist. The effects of the diameter and the amount of mist upon the heat transfer enhancement were investigated analytically and experimentally. The images of the flow field and the behavior of the mist were taken using a high-speed video camera. The distributions of spatial temperature and the heat transfer coefficient were measured. The evaporation position of the mist in the main flow was calculated with the changing parameters of the relative humidity and the particle diameter of the mist. The results of the visualization experiment and the theoretical calculation revealed that the mist almost evaporated in a relatively small region of the channel. The experimental results show that the heat transfer coefficient becomes maximum near the reattachment position irrespective of the amount of mist, and that the maximum and the average Nusselt numbers increase linearly as the amount of mist increases.