Abstract: Taking water as a medium,the experimental study on the heat transfer and flow resistance of three flat tubes with different geometric sizes is carried out.Experimental results show that the flat tube and circular tube with the same perimeter are with the same Reynolds number under the same water flow rate,however the heat transfer coefficient of the three flat tubes is higher than that of the circular tube and the larg-est heat transfer coefficient of the former can be 2.43 times of the latter.It is concluded that the flat tube is with a good heat transfer enhancement effect.Pressure drop of the latter is however higher than that of circu-lar tube.Finally,the heat transfer performance of horizontally and vertically laid flat tube in the casing pipe is studied and compared,and the experimental result indicates that the vertical one gains the advantage over the other.