This experimental study aims to investigate the heat transfer phenomena of an outer tube with an inner-tube insertion. The naphthalene sublimation method is adopted which measures the sublimation depth of naphthalene necessary to reduce the local heat transfer through the analogue relation between heat and mass transfer. The working fluid is air and the data runs are performed for Reynolds numbers of 1058, 1360 and 1965. The comparison between experimental and numerical results shows good agreement. It is also found that, with an inner-tube insertion, the heat transfer rate of the outer tube increases as the Reynolds number of the tube flow and the size of inner tube increase, as long as the inner tube is not Larger than a given size.