ZIF-8 nanoparticles were doped into P84 polymer matrix to synthesize defect-free mixed matrix membranes (MMMs) for CO2/CH4 separation. A remarkable combination of gas permeability and separation factor appeared in MMMs with filler loading of 27 wt%. The permeability of CO2 achieved 10.92 Barrers along with CO2/CH4 separation factor of 92.6, separately corresponding to an increment of 339% and 35.7% compared with the pure P84 membrane, which breaks the trade-off between permeability and selectivity of polymer membranes. Adsorption and diffusion coefficient of CO2 and CH4 for ZIF-8/P84 MMMs as well as the apparent activation energy of the corresponding processes were quantitatively determined, indicative of a synergetic effect of the selective adsorption and molecular sieving property arising from ZIF-8 nanoparticle fillers on the significant improvement of separation performances.