In this research, the heat transfer coefficient (HTC) and pressure drop of alternating cross-section flattened (ACF) tubes were investigated experimentally and compared with the same parameters of a circular tube. Three different ACF tubes were fabricated from circular copper tubing with an internal diameter of 4.5 mm, a thickness of 1 mm, and a length of 400 mm. The twist angles were 30, 45, and 90°. The experimental ranges covered a mass flux of 729–1,434 kg/m2s and a heat flux of 12–30 kW/m2. The results showed that the HTC and pressure drop increased with mass flux. The HTC decreased with increments of heat flux, but the pressure drop did not change. The HTC and pressure drop of the ACF tubes were higher than those of the circular tube. The ACF tube with 90° twist angles produced the highest HTC, and the thermal performance of that tube was about 27% better than the thermal performance of the circular tube.