The numerical simulation of turbulent heat transfer for helium gas flowing in a horizontal small-diameter tube was performed. The numerical simulation conditions were 0.8 mm and 1.8 mm for inner diameters, and 102-128 m/s and 87-241 m/s for velocities at the inlet of the tube, respectively. The numerical solution was carried out by applying ANSYS FLUENT software. The standard k-ε model and a user-defined wall function were used for the analysis. The thickness of the conductive sublayer, which is the region where heat transfer by conduction is dominant, was estimated. As a result of the numerical simulation, it was found that the thickness of the conductive sublayer becomes thinner as the flow velocity increases and the inner diameter decreases. Moreover, the heat transfer coefficients of the simulation data were compared with the previous experimental data, and they were in good agreement with the experimental data.