The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La₂Hf₂O₇ nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition.We have discovered that in our La₂Hf₂O₇:U NPs, the uranium dopants are stabilized at both U⁴⁺ and U⁶⁺ oxidation states in which the U⁶⁺ oxidation state exists in octahedral uranate UO₆^6- form. We also confirmed that the U⁴⁺ ions substituted the Hf⁴⁺ ions with a lifetime of _˜1.0 µs and the UO₆^6- ions resided at the La³⁺ sites with a lifetime of _˜9.0 µs. More interestingly, the proportion of the U⁴⁺ ions in the La₂Hf₂O₇:U NPs was higher than that of the UO₆^6- ions at low doping level, but at the doping level higher than 2.5%, the fraction of the UO₆^6-  ions was greater than that of the U⁴⁺ ions. Furthermore, we studied the structural phase transformation from order pyrochlore to cotunnite of these La₂Hf₂O₇:U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U⁴⁺ ions whereas disordered cotunnite phase favors the UO₆^6- ions. We further used in situ Raman spectroscopy to confirm the reversible cotunnite to pyrochlore phase transformation of the La₂Hf₂O₇:10%U NPs at 900 °C. Therefore, this work demonstrated the successful development of uranium doped La₂Hf₂O₇ NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase transformation, and structure–optical property correlation.