A new expression has been developed for prediction of the optimal fin spacing for vertical rectangular fins protruding from a vertical rectangular base. This expression was determined using the results of experimental investigation available in the literature over a wide range of test variables. Data collated from literature covered the range of fin spacing from 2.85 mm to 85.5 mm. The range of basc-to-ambient temperature difference was quite extensive, from 14 degrees C to 162 degrees C. The fin length range was from 100 mm to 500 mm. the fin height from 5 mm to 90 mm, the fin thickness from 1 mm to 19 mm, the width of rectangular base plate from 180 mm to 250 mm. Its dimensionless parameters were formulated by the intersection of asymptotes method proposed in (Yildiz, and Yuncu, 2004). These dimensionless parameters correlate the experimental data of seven investigators. The expression resulting from this correlation predicted the experimental data within an average overall error of less then 24 percent.