The horseshoe vortex structure upstream of tubes in a two-row plate-fin-and-tube heat exchanger was visualized by means of a PIV system (Particle Image Velocimetry). The effect of the fin spacing and the flow velocity were both examined. Three fin spacings were tested for a Reynolds number, ReD, ranging from 280 to 1580. It was observed that the horseshoe vortex structure may form in one or two major vortices depending on the fin spacing and the flow velocity. Due to the development of the boundary layer on the fins, the horseshoe vortices upstream of the tubes of the second row are more noticeable than those upstream of the tubes of the first row.