An experimental investigation was conducted to determine the influence of fin aspect ratio on the heat transfer enhancement in narrow, rectangular ducts. Experiments were carried out in a transparent flow channel of rectangular cross section using an inline array of 24 short, square fins mounted on the bottom wall. The fin length-to-height aspect ratio (L/B) was varied over three values: 2, 1, and 0.67. The hydraulic diameter-based Reynolds number was varied from 800 to 8500. The experiments were conducted in water and a dielectric liquid.The results obtained showed an increase in pressure drop as the fin aspect ratio decreases. However, the nondimensional heat transfer coefficient for the three-dimensional fins of this study was found to be independent of fin aspect ratio; this is in contrast to the expected behavior based on information in the literature for two-dimensional ribs. An empirical correlation is proposed for the heat transfer coefficient as a function of Reynolds number and fluid properties.