The mixing efficiency in a curved tube is a complex function of the Reynolds number, Schmidt number, curvature ratio and tube pitch, therefore, the relative effectiveness of a helical tube is quite complicated and challenging over a straight tube. A state of art review on mixing of two miscible liquids in curved tubes revealed that the mixing in coils of circular cross section has not been reported in the literature. In the present work a computational fluid dynamics study is performed in curved tubes of circular cross-section of finite pitch under laminar flow conditions to examine the scalar mixing of two miscible fluids using scalar transport technique. In the present study the phenomenon of mixing by convection and diffusion of two flow streams with inlet scalar concentrations of zero and unity in the two halves of a tube perpendicular to the plane of curvature has been reported. The mixing efficiency has been quantified with concentration distributions and unmixedness coefficient at different cross-sections and process conditions (Reynolds number, Schmidt number and curvature ratio) in the straight and curved tube of circular cross-section. The result shows that, in curved tube, for higher Schmidt number fluids, mixing is considerably improved at moderately low Reynolds numbers (Re similar to 10), but is not affected for Reynolds number of the order of 0.1. It is also reported that mixing in the curved tube is higher at low values of curvature ratio as compared to the higher curvature ratio. (c) 2006 Elsevier Ltd. All rights reserved.