Porous single -layer graphene is promising as the selective layer for membrane-based gas separation thanks to the atomic thickness of the pore which can yield high permselective flux. Direct synthesis of porous graphene by chemical vapor deposition (CVD) is highly attractive to reduce the number of steps in membrane fabrication. This has been demonstrated in the past by incorporating pores as grain-boundary defects, however, pore density in graphene has been limited because of challenges in increasing grain density of single layers to nanocrystalline regime. Herein, we systematically tune the CVD conditions including growth temperature, methane partial pressure and methane/hydrogen ratio to find a low-temperature growth regime where continuous unfragmented graphene film could be synthesized while avoiding multilayers. The resulting graphene is nanocrystalline composed of misoriented nanometer-scale grains with a high density of hydrogen-permeable multivacancy defects or pores. Centimeter-scale single -layer porous graphene membranes yield extremely high H2/SF6 selectivity, reaching above 1000, confirming the high-quality of porous graphene with pores smaller than 0.55 nm, consistent with the structure and distribution of vacancy defects revealed with microscopy.