Abstract: Two kinds of lanthanide metal-organic frameworks such as terbium benzene-1,3,5-tricarboxylate (Tb(BTC)) and europium benzene-1,3,5-tricarboxylate (Eu(BTC)) were synthesized by hydrothermal method. 1,4-bis(2-trifluoromethyl-4-aminophenoxy)benzene (6FAPB) and 1,2,3,4-cyclobutane dianhydride (CBDA) as the monomers of polyimide (PI), the Tb(BTC)/PI and Eu(BTC)/PI composites containing 7wt% Tb(BTC) or Eu(BTC) (mass fraction) were prepared by the in-situ polymerization. The structure and properties of the Tb(BTC)/PI and Eu(BTC)/PI composites were characterized by FTIR, UV-visible spectrometer (UV-vis), TGA, XRD, SEM, universal tensile testing machine and gas permeabilities testing. These experimental results show that Tb(BTC) and Eu(BTC) contain very few pore structures, belonging to the mesoporous range, and exhibit high thermal stabilities. The introduction of Tb(BTC) and Eu(BTC) improves the thermal and mechanical properties of PI(6FAPB-CBDA). The glass transition temperatures increase from 351.9℃ of pure PI(6FAPB-CBDA) to 358.0℃ and 354.8℃, and the 5% thermal decomposition temperatures increase from 431.6℃ to 447.8℃ and 441.1℃. The tensile strengths of Tb(BTC)/PI and Eu(BTC)/PI composites increase from 60.8 MPa to 77.7 MPa and 70.4 MPa, and the Young's modulus increase from 1.54 GPa to 2.80 GPa and 2.17 GPa, respectively. However, the optical transmittances at 500 nm of Tb(BTC)/PI and Eu(BTC)/PI composites reduce from 82.3% to 23.0% and 24.2%. The results of gas permeability testing show that both of Tb(BTC) and Eu(BTC) can improve the gas permeabilities of the PI-(6FAPB-CBDA) membrane. The effect of Eu(BTC) is better than Tb(BTC). The permeabilities of H₂, O₂, N₂ and CO₂ of Eu(TBC)/PI composite are 119.23, 15.02, 3.21 and 90.35 Barrer, respectively, and O₂/N₂ is 4.68, CO₂/N₂ is 28.15.