Thanks to the high heat transfer performance and compact structure, the finned tube bundles with enhanced slotted plain fins have received more and more attention in the air-cooled heat exchangers of natural draft dry cooling system in power plants. In this paper, two kinds of rectangular slot configurations, continuous slots and alternating slots, were presented and the effects of the slots on the air-side thermo-flow performances of the plain finned tube bundles in in-line and staggered configurations were analyzed by means of numerical simulations, which were validated by the experiment. The performance evaluation criterion was applied to predict the improvement of the thermo-flow performances of plain finned tube bundles by slot configurations. The results show that the slot on the fin surface can cause a considerable heat transfer augmentation of finned tube bundles with a modest pressure loss penalty when compared to the plain fin. However, the slotted fin only improves the heat transfer for the main-flow zones. Due to the relatively drastic swirled flow generation, the slotted fin with continuous rectangular strips shows a better performance than the alternant upper and lower slotted fin, although the latter has a higher heat transfer capability. For the overall performance, the finned tube bundles in a staggered pattern are superior to those in an in-line one. The staggered fin-and-tube bundles with continuous slotted rectangular strips possess best overall performance, so are recommended to apply to the air-cooled heat exchanger. (C) 2015 Elsevier Ltd. All rights reserved.