Liquid ice is useful as a cold heat storage material. In this work, heat transfer experiments in U-bent pipes for continuous formation of liquid ice were carried out to make clear the effects of pipe diameter on heat transfer efficiency and mass fraction of ice. In the case of using a 15-mm pipe, the experimental results showed that the mass fraction of ice and the heat transfer efficiency had maximum values at a certain volume fraction of water and that the heat transfer efficiency of a 15-mm diameter U-bent pipe system was about twice as large as those of 12-mm and 10-mm diameter pipe systems. From photographs of flow visualization, it was found that these phenomena were caused by the difference of flow pattern in the downward and horizontal flow region of the U-bent pipes.