Metal-organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass-transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. A modified soft-physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H2 /CO2 separation performance, with a separation factor of up to 166 and H2 permeance of up to 8×10-7  mol m-2  s-1  Pa-1 at elevated testing temperatures owing to a well-defined size-exclusion effect. This nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.