Flow and heat transfer in a plate fins/circular tube assembly is examined using naphthalene sublimation technique. The examined parameters are the gap (6) to tube diameter (D) ratio delta/D. the Reynolds number Re-D and the tube location l/D for a single tube. A preliminary flow visualization shows large recirculating twin vortices and a weak downstream oscillatory streak-line. The local heat/mass transfer coefficient is large at the leading edge of the plate and also in front of the tube. It is relatively small behind the tube and it approaches the fully developed asymptotic value far downstream. The high heat/mass transfer coefficient in front of the tube is considered to be due to the so-called horseshoe vortex. When the Reynolds number is as large as 2660, a smaller subsidiary horseshoe vortex is attached to the upstream of the main one. The positive effect of the horseshoe vortices is prominent when the tube is placed in the downstream region. In this case. the total heat/mass transfer rate increases up to 25%.