A thermal-hydraulic study of an isolated Advanced Gas-Cooled Reactor (AGR) nuclear fuel rod with smooth and rough cladding surfaces is carried out by computational fluid dynamics simulation and analytical calculation. Square transverse ribs of various pitch/height ratios (6:12) are considered for the rough surface. Parameters of the rough cladding surface show greater values than those for the smooth surface except for surface heat flux. It is found that only the average surface heat flux increases with an increasing pitch/height ratio. On the other hand, the average values of wall shear stress, Darcy friction factor, skin friction factor, convective heat transfer coefficient, Nusselt number, and thermal-hydraulic performance decrease with an increasing pitch/height ratio. The simulated results are found to be very close to the values obtained from an analytical calculation. Also, square and circular ribs are compared. The circular ribs show lower values of convective heat transfer coefficient and wall shear stress but permit high surface heat flux. The results of this study will help researchers comprehend the effect of cladding surface roughness on fuel rod thermal behavior.