Tube-in-tube heat exchanger has been widely used in heat pump systems including transcritical carbon dioxide system. To improve system compactness, the tube inside the exchanger is usually spirally coiled in actual application while the conventional focus of most existing studies is straight tube. In this study, coiled tube of a tube-in-tube heat exchanger in a transcritical carbon dioxide heat pump cycle is experimentally investigated. Fluid temperature and pressure were measured at different state points on the heat exchanger under different refrigerant charge conditions. Theoretical analysis was performed basing on the Darcy–Weisbach equation. The performance of straight tubes is predicted theoretically and compared with the performance of coiled tube.