Heat transfer in laminar flow between two parallel plates with repeated rib-type eddy promoters was studied numerically. It has been found that the heat transfer is enhanced with respect to the flat plate channel flow when the nondimensional clearance between the promoter and the heated duct wall is larger than 1.0. The highest value of the averaged Nusselt number obtained in the present study is 23 which is about 4.3 times the constant value of 5.385 typical of smooth duct flow. The observed heat transfer enhancement is due to 1. Transport of the relatively low-temperature main fluid body into a region close to the heated wall, and 2. Flow mixing which results in a more uniform temperature distribution across the main flow field.