Developing porous adsorbents for efficient separation of C 4 olefins is significant but challenging in the petrochemical industry due to their similar molecular sizes and physical properties. The separation efficiency is often limited when separating C 4 olefins by a single separation mechanism. Herein, an ultramicroporous yttrium-based MOF, Y-dbai, is reported featuring cage-like pores connected by small windows, for recognition and efficient separation of C 4 olefins through a synergistic effect of thermodynamic and kinetic mechanisms. At 298 K and 1 bar, the adsorption capacities of Y-dbai for C 4 H 6 , 1-C 4 H 8 , and i-C 4 H 8 are 2.88, 1.07, and 0.14 mmol g -1 , respectively, indicating a molecular sieving effect toward i-C 4 H 8 . The C 4 H 6 /i-C 4 H 8 and 1-C 4 H 8 /i-C 4 H 8 uptake selectivities of Y-dbai are 20.6 and 7.6, respectively, outperforming most of the reported adsorbents. The static and kinetic adsorption experiments coupled with DFT calculations indicate the separation should be attributed to a combined effect of thermodynamically and kinetically controlled mechanism. Breakthrough experiments have confirmed the excellent separation capability of Y-dbai toward C 4 H 6 /1-C 4 H 8 , C 4 H 6 /i-C 4 H 8, and C 4 H 6 /1-C 4 H 8 /i-C 4 H 8 mixtures.