Convective heat transfer and single phase water flow in concentric annular tubes having narrow gap were numerically and experimentally investigated. The inner diameter of copper tube having 1mm wall thickness is fixed 18 mm. The rodes having diameter of 16, 17 ye 17.5 mm are concentrically inserted in the tube. The gap of the annular duct is fixed at 1, 0.5 and 0.25 mm. The numerical study was based on the Finite volume method. Reynolds Stress model is used for turbulent region. The Reynolds number was chosen in the range of 600-12000. The results are compared with the conventional correlations presented for macro annular ducts in literature. The results show that heat transfer and single phase water flow in mini/micro annular ducts are different from those of macro-sized annular ducts for turbulent regime as it is in good agreement in laminar regime. For laminar and turbulent flow, the ratios of experimental to numerical values are found about 0.97 - 1.01 and 1.08 - 1.3 for friction factor and 1.05-1.17 and 0.97 - 1.24 for Nusselt number, respectively. For turbulent flow, the numerical and experimental values are found bigger about 9-36% and 18-36% for Nusselt number and 23-43% and 45-69% for friction factor than conventional results presented in literature for macro-sized annular duct, respectively.