A three-dimensional numerical simulation of the heat transfer characteristics of single-phase liquid nitrogen flowing through heated vertical U-tube was conducted. The structured grid was used and the mesh near the wall was refined. Besides, the grid independence and model availability was verified. The velocity inlet, pressure export and uniform wall heat flux were used as boundary conditions. In addition, the standard k-ε turbulence model was adopted and solved by CFX 4.4. It is found that the fluid temperature is lower in the center and higher near the wall in the U-bend upstream pipe, while the fluid temperature is reconstructed by the secondary flow in the U bend: the cold core is flowing to the concave surface, and the high temperature fluid is close to the convex surface; and in the downstream of the pipe, the temperature distribution is restored. The impact of curvature ratio on the downstream Nu number is significant and the affected area can be measured by the minimum Nu value. There is a curvature ratio which makes the percentage of the downstream to the upstream Nu to be the minimum. Finally, the comparison of simulation results and the correlation was conducted.