Heat transfer enhancement in the ducts is significantly related with flow separation and reattachment regions. Therefore, ribs are used to obtain the rotational flows in the vicinity of the wall since the fluctuations in the thermal and hydrodynamic boundary layers are effective for the increment of heat transfer by convection. Even though heat transfer surface area is enlarged by placing the ribs into the channels, the pressure loss due to the ribs has to be taken into account and controlled in these systems. Based on the aforementioned explanations, the square cross-sectional ribs have been mounted on the bottom and the top walls of horizontal parallel plates in terms of staggered arrangement. In the present paper, numerical analyses have been conducted via k-ω SST turbulence model at Re = 10000, 15000 and 20000 for different spacing values between two successive ribs. For the constant rib dimensions, the ribbed models have been compared by referring to smooth plates as reference model. For this reason; time-averaged results including streamwise velocity components, temperature distributions, pressure values, streamline patterns and Nusselt numbers for the ribbed and the smooth plates have been separately presented and compared. Heat transfer has been enhanced due to the ribs deforming the hydrodynamic and thermal boundary layers. Furthermore, increasing the rib spacing and Reynolds number has enhanced Nusselt number. As observed, heat transfer has also been augmented by extending the distance between two successive ribs since fluid easily penetrates to the sink between them for the production of recirculation regions. However, in case of the ribbed models, there are gradual drop pressure values along the channel. For this reason, the rib effect on these values is much more as evidently observed at Re = 10000.