A step heating method for the measurement of the thermal diffusivity of diamond thin film is described. The step heating method is a transient heat flow method. Transient temperature profiles are generated in a strip-shaped sample by heating one end of the sample while the other end is clamped to a heat sink. Three thermocouples are used along the heat path. The results are compared with the literature values over the temperature range from –190 to 50°C.