The thermodynamic quantities (ΔG, ΔH, ΔS) of complex formation of U(VI) with three "aliphatic" di-carboxylic acids, i.e. oxydiacetic acid (ODA), thiodiacetic acid (TDA), iminodiacetic acid (IDA), and three "aromatic" di-carboxylic acids, i.e. 2,5-furandicarboxylic acid (FDA), 2,5-thiophenedicarboxylic acid (TDCA) and dipicolinic acid (DPA) were determined by potentiometric titration and micro-calorimetric titration. The obtained thermodynamic quantities are compared with each other for the discussion of the effect of structural difference, i.e. aliphatic- and aromatic structure and types of central atom in the ligands on the complex formation thermodynamics. A clear tendency is observed in the complexation data between aliphatic- and aromatic ligands. The complexation of those aliphatic ligands are completely entropy driven, on the contrary, the enthalpy change is a certain driving force in the complexation of those aromatic ligands while entropy change also contributes to progress of the reaction. The ΔG, ΔH and ΔS of 1:1 IDA complex show remarkable difference from those of glutaric acid (GA) complex, while those of ODA and TDA complex are almost equal to those of GA complex, which seems to be attributed to the difference of the coordination mode.