In this study, the heat flow characteristics of natural convection was theoretically analyzed with time by changing various locations of heat trace with tube surface about enclosed circular tube by applying nominal tube diameter 90 mm of KS D 3507. Before carrying out CFD (Computational Fluid Dynamics) analysis, it is presumed that the boundary condition is the tube’s inside and outside fluid temperature of 273 °K, the tube surface heat flux according to the heat trace location of 16 W/m. The result confirmed that water of inside tube is occurred natural convection caused by density difference depending on temperature profile. Additionally, in case of heat trace location, the heat transfer and flow characteristics showed clearly that two-heat trace location =135° and 180° was more favorable distribution than one-heat trace appropriate location =135°.