The effect of thermal boundary conditions on heat transfer was investigated experimentally with sodium flowing through a tube by controlling axial heat flux distribution. The range of sodium temperature was from 190 to 280 deg C, and the range of Peclet number was from 110 to 400. The Nusselt number in a developed region where heat flux was constant axially, was in good agreement with an accepted empirical formula. In the prior section of the tube, it increased with the Peclet number. When the heat flux distribution was made exponential axially, the tube-wall temperature finally attained constant value in the developed region, where the Nusselt number was also constant. The Nusselt number in this case was lower than that in the constant heat flux. In sinusoidal heat flux distribution, the local Nusselt number varied with the Peclet number, but it was not much influenced by L/D ratio. (JA)