Abstract: It is important in the experimental investigations of convection heat transfer to carry out the experiments under known fluid thermal boundary conditions. In this paper, in order to identify the influences of wall heat conduction on the fluid thermal boundary conditions, the conjugate heat transfer problem of wall-heat-conduction and forced convection in the thermal entrance region of fully-developed laminar pipe flows with non-uniform heating is studied numerically to find out the influences of pipe sizes, fluid and pipe material properties and flow states on the homogeneity of the fluid thermal boundaries at the fluid-wall interface. The results show that the conjugate heat transfer between the tube wall and fluid has a strong influence on the boundary condition of the in-tube fluid at the wall. Suggestions are given on the size and the wall material of the test section to improve the reliability and accuracy of experimental results.