Holographic interferometry is utilized to qualitatively study the effects of the horizontal fins on natural-convective heat transfer performance over a heated vertical plate, which forms a square enclosure with a cooled vertical plate and nonconductive wads. A laser light sheet is employed to Issueless the flow field over the heated vertical plate with or without fins. It is disclosed that (1) convective heat transfer performance deteriorates with the installation of fins shorter than the boundary-layer thickness delta on the vertical plate, but the average Nusselt number enhances with on increase in the fin length and reaches as much as 1.3 to 1.7 times that without fins: (2) There is an optimum fin length to enhance the average heat transfer performance. (3) The upward-facing surface (downstream side) of the fins performs better convection heat transfer than the downward-facing surface (upstream side). mow visualization reveals that fluid flow separates from the upward-facing surface of the fin tip and turbulent flow occurs inside the boundary layer behind the protruding tip. (4) Coanda-effect flows are observed at the restricted portion, that is, at the bottom corner of the heated surface, and moreover on the condition including the fin of a 20-mm height above the floor. Turbulent-effect flows are observed everywhere if there is no fin at the position of a 20-mm height above the floor. (5) The convective heat transfer resulting in the turbulent mechanism is superior to that resulting in the Coanda effect.