Mixed convection of air with Pr = 0.7 between two horizontal concentric cylinders which are held at different uniform temperatures is-numerically investigated. The forced flow is induced by the cord outer cylinder which is rotating slowly with constant angular velocity with its axis at the center of annulus. Investigations are made for various combinations of Ra, Re and a (= diameter of inner cylinder/gap width) in the range of Ra less than or equal to 5 x 10(4), Re less than or equal to 1500, and 0.5 less than or equal to sigma less than or equal to 5. The flow patterns can be categorized into three basic types according to the number of eddies : two-eddy, one-eddy, and no-eddy flows A map of the three flow regimes is constructed on the Ra-Re plane. Characteristics of flow patterns and heat transfer are elucidated. (C) 1997 Elsevier Science Ltd.