The microstructural changes and associated effects on thermal conductivity were examined in UO 2 after irradiation using 3.9 MeV He 2+ ions. Lattice expansion of UO 2 was observed in x-ray diffraction after ion irradiation up to 5×10 16 He 2+ /cm 2 at low-temperature (< 200 °C). Transmission electron microscopy (TEM) showed homogenous irradiation damage across an 8 µm thick plateau region, which consisted of small dislocation loops accompanied by dislocation segments. Dome-shaped blisters were observed at the peak damage region (depth around 8.5 µm) in the sample subjected to 5×10 16 He 2+ /cm 2 , the highest fluence reached, while similar features were not detected at 9×10 15 He 2+ /cm 2. Laser-based thermo-reflectance measurements showed that the thermal conductivity for the irradiated layer decreased about 55 % for the high fluence sample and 35% for the low fluence sample as compared to an un-irradiated reference sample. Detailed analysis for the thermal conductivity indicated that the conductivity reduction was caused by the irradiation induced point defects.