Mixed convection flows in concentric curved annular ducts with constant wall temperature boundary condition are studied numerically. The flow is assumed to be fully developed so as to maintain a constant streamwise pressure and temperature gradient. Key parameters for the flow are the radius ratio (ratio of the inner core radius to the outer pipe radius), the Dean number, and the Grashof number. Computations are carried out for flows of various radius ratio with Dean numbers in the range 0-900 and Grashof numbers of 12.5 and 12 500. The secondary flow patterns, the streamwise velocity profiles and the heat transfer coefficients are presented. Effects of the Dean number and the Grashof number on the flow pattern, on the friction ratio (ratio of the friction for a curved annular duct to that for a straight annular duct flow), and on the heat transfer property are discussed based on the computational results. It is found that both the friction ratio and the Nusselt number ratio are strong functions of the radius ratio and the Dean number when Gr = 12.5, while these ratios do not change much with the radius ratio and the Dean number when Gr = 12 500.