Abstract: The region from cladding to pellet of the leak rod with the burnup of 41 GW·d/tU and the intact rod with the burnup of 45 GW·d/tU was characterized by nanoindentation, metallurgical microscope, and Raman spectroscopy. The results show that a porous high burnup structure (HBS) appears on the pellet periphery of the intact rod, and this phenomenon results in the decrease of the hardness of periphery of the pellet. The hardness of the pellet of the leak rod decreases along the radial direction towards the centre of the pellet, which is mainly related to the migration of the pores and the reconstruction of the fuel pellet. After the leak of the cladding, coolant enters the gap between the cladding pellets and reacts with the pellets. The porosity of the HBS of the leak rod is less than that of the intact rod, and the hardness of the HBS of the leak rod is slightly higher. Meanwhile, the temperature change leads to grain reconstruction and the formation of large-sized columnar crystals, resulting in the decrease of hardness in the middle and centre of the leak rod fuel pellet. The leak of the cladding does not change the phase composition of the pellets. As a result, the Young's modulus of intact rod and leak rod pellets does not show much difference. The hardness of the FCCI layer of the intact rod increases gradually along the cladding towards the pellet, while the Young's modulus firstly decreases and then increases. The hardness of the FCCI layer of the leak rod increases at first and then decreases, while the change of the Young's modulus is in general similar to the hardness of the leak rod along the cladding to the pellet. The phenomenon arises from the combined effects of phase structure transformation and fission product doping. The hydrogenation and oxidation of the cladding after the leak of the fuel rod results in a significantly higher hardness and Young's modulus of the leak rod cladding than that of the intact rod.