This paper presents results of an experimental program to determine the leakage class for three cross section flat oval ducts for positive and negative internal pressures. Sealed and unsealed duct leakage tests were performed. The leakage data were found to be a power law function of static pressure difference between the interior and the exterior of the duct. The power law exponent ranged from 0.675 to 1.065 for sealed conditions, and 0.437 to 0.729 for unsealed conditions. Generally the exponent found for an individual duct tested in the negative pressure mode was less than that for the corresponding positive pressure mode. Leakage class values ranged from 0.034 to 1.048 mL/s per m² (0.23 to 2.2 cfm per 100 ft²) for sealed duct tests, and between 297.4 to 1342.2 mL/s per m² (197.2 to 366.5 cfm per 100 ft²) for unsealed tests where sheet metal screws were not used to assemble the duct sections. The flat oval sealed leakage results were lower than those for spiral and longitudinal seam circular ducts reported in the ASHRAE Handbook.