Velocity and temperature fields of fully developed flow in concentric curved annular ducts are studied numerically. Computations are carried out for the forced convection flows with the Dean number lying in the range 0<k<900, and the radius ratio in the range from 0.1 to 0.9. Secondary flow patterns are illustrated by streamlines, and the friction and heat transfer properties for curved annular duct flows are explained in comparison with those for straight annular duct flows. It is observed that the friction ratio and the overall average Nusselt number are proportional to κ<sup>½ for the wide range of the Dean number considered here.