For the first time, boron embedded carbon membranes have shown superior C2H4 permeability and C2H4/C2H6 selectivity to the upper bound line under both pure and mixed gas tests. The precursors of the carbon membranes were prepared by incorporating various boron compounds with different molecular sizes into a hydrolyzed polymer of intrinsic microporosity (PIM-1) and then carbonized at 700 degrees C. The increments in C2H4 permeability and C2H4/C2H6 selectivity followed the order of molecular size of the boron compounds. Both PALS results and CO2 adsorption isotherms confirmed the enlarged pore size and increased pore amount. The enhancements in C2H4 permeability and C2H4/C2H6 selectivity were mainly due to the higher C2H4 diffusivity and greater diffusion selectivity resulting from proper choices of boron additives and carbonization conditions. When 9,9dihexylfluorene- 2,7-diboronic acid (DHFDA) was embedded into the hydrolyzed PIM-1 and then carbonized at 700 degrees C, the C2H4 permeability and C2H4/C2H6 selectivity of the carbonized membrane under mixed gas tests increased from 7.5 to 13.7 Barrer and from 9.3 to 9.7, respectively.