Abstract: The tendency of third phase formation (TPF) in amides extraction process of U(Ⅵ) is very different for different kinds of amides. The order of the tendency of TPF is namely DB2EHA with a-branching of carbanyl chain >DB2MBA with β-branching of carbanyl chain~DsBHA with branching of ni-trogen>DBOA with long straight chain structure. Addition of a little TBP can decrease the tendency of TPF.There is no TPF for DBOA under very high uranium concentration. The composition of extraction complexes, either with TPF or without, is determined and it is shown that the same molar ratio of amides to UO2 2+ exists in the extraction complex, namely two molecules of amides to one molecule of UO2(NO3)2. Besides, there may be another extraction complex consisting of 3 molecules of amides and 2 molecules of uranyl nitrate.