Abstract: In liquid-liquid extraction of the spent nuclear fuel reprocessing, the organic phase will usually split into a heavy phase and a light phase, and the heavy phase is well known as the third phase. Formation of the third phase restricts the stable and continuous operation in the reprocessing of spent fuel, and can potentially cause safety and even criticality concerns due to the high Pu(Ⅳ) concentrations. Great efforts have been devoted to investigate the process of the third phase formation since the foundation of nuclear industry, and it is now known that a series of parameters such as the temperature, acid, extractant, diluent, etc. affect the third phase formation. The formation of the third phase is often believed to be the result of limited solubility of metal-ligand complexes in the organic diluent, however, the exact molecular interpretation remains largely unknown. Since 1970s, the concept of colloidal chemistry has been introduced to further illustrate the third phase, growing experimental evidences indicate that the formation of the reversed micelles or micro-emulsions are responsible for the phase transition process. And this model based on the amphiphilic molecule self-assembly can satisfactorily explain the third phase formation. The recent progress of the mechanistic studies of the third phase formation concerning the spent fuel reprocessing on the basis of colloid chemistry is reviewed.