A three-dimensional numerical investigation was carried out to clarify the air-side flow and heat transfer performances of flat and slotted finned tube bundles, which was validated by the experimental test. For both the flat and slotted fin geometries, fifty models of finned tube bundles at various longitudinal tube pitches (25 mm, 30 mm, 35 mm, 40 mm and 45 mm) and transverse tube pitches (25 mm, 30 mm, 35 mm, 40 mm and 45 mm) were developed and analyzed at different air inlet velocities, based on which the correlations for f, Nu and P-EC were obtained respectively. The results show that the finned tube bundles with slotted fins are superior to those with flat fins in heat transfer performance, but present a high pressure drop. The friction factor varies little with the tube pitch, however the Nusselt number decreases conspicuously with increasing the longitudinal and transverse tube pitches. The variations of the performance evaluation criterion with the transverse and longitudinal tube pitches show that better thermo-flow performances can be achieved by reducing the tube pitch. From the actual needs of engineering, the optimal configurations of the finned tube bundles with the tube pitches from 35 mm to 40 mm are recommended for air-cooled heat exchangers by using the system performance criterion. (C) 2016 Elsevier Ltd. All rights reserved.