Abstract: Ionic liquid can participate in or affect the self-assembly of amphiphiles due to its unique physical and chemical properties. Most amphiphiles used in the field of self-assembly based on ionic liquid are organic compounds, while the assembly of metal complexes in ionic liquid is rarely reported. In addition, the complex formed in the extraction of UO²⁺₂ by octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide(CMPO)-1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(C₂mimNTf₂) system needs further study. In this work, the self-assembly of UO₂(CMPO)₃(NO₃)₂ in C₂mimNTf₂ was studied. In situ transmission electron microscope(in situ TEM) demonstrats that UO₂(CMPO)₃(NO₃)₂ formed assembly in C2mimNTf2 in the presence of 70 μL water. Freeze-fracture transmission electron microscope(FF-TEM) shows that the assembly was micelle.In addition, the complex formed in the extraction of UO²⁺₂ by CMPO-C₂mimNTf₂ system was studied. Ion chromatography shows that the concentration of NO^-₃ in water phase is almost unchanged before and after extraction. The peaks appeared in electrospray ionization mass spectrometry(ESI-MS) are attributed to the fragments of UO₂(CMPO)₃(NTf₂)₂. These results indicate that the extraction complex is UO₂(CMPO)₃(NTf₂)₂ instead of UO₂(CMPO)₃(NO₃)₂. This work helps understand the assembly of metal complexes in ionic liquid and provides an important insight into the extraction mechanism of UO²⁺₂ by CMPO-C₂mimNTf₂ system.