Three Zircaloy-clad UO2 test fuel rods were irradiated at 50 kW/m to a burnup of 15 000 MWd/MTU with an in-pile rod diameter scanning device, and cladding deformations were measured at various power and bumup levels. Major mode of deformation was ridging at pellet interface positions. Magnitude and relevant power levels for various modes of cladding deformation were found to be strongly dependent on initial pellet-cladding gap size. During the first approach to power, smaller gap size caused larger cladding deformation; however, the effect was reversed at higher burnup. Cladding diametral deformation was found to be dictated not only by local thermal expansion of a single pellet, but also by long-range axial force in the pellet stack. In the first power cycle, cladding deformations caused by power ramp were found to decrease during subsequent power holding, but their rate was much reduced at high burnup.