The heat transfer of fully developed laminar flow in internally finned tubes is investigated analytically. If there is no heat generation in the fluid, the highest Nusselt number is obtained for the tube with 22 fins extended to about 80 percent of the tube radius. Its value is almost 20 times that for the finless tube. When there is heat generation at sufficiently large rate, the number of fins is reduced from 22 to 16 in order to obtain the highest Nusselt number.