This study first develops a facile method to synthesize zeolitic imidazolate framework cuboid (ZIF-C) nanosheets with tunable thickness from 70 to 170 nm from aqueous polymer solutions. The obtained ZIF-C nanosheets were characterized by various techniques, including X-ray diffractometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N2 adsorption and thermogravimetric analysis (TGA), to understand their compositional and structural properties. The synthesized ZIF-Cs nanosheets with different thicknesses were further applied as nanofillers to prepare Pebax-based mixed matrix membranes (MMMs) to study the effect of the morphology on membrane properties and CO2/N2 separation performances under different relative humidity (RH) conditions. Results reveal that the incorporation of these ZIF-Cs simultaneously enhances CO2 permeability and CO2/N2 selectivity in the mixed matrix membranes. In addition, MMMs with the thickest ZIF-C nanosheet present better performance. A CO2 permeability of 387.2 Barrer accompanied with a CO2/N2 selectivity of 47.1 has been documented, nearly doubled in CO2 permeability with slightly increased selectivity compared with membranes containing thinner nanosheets.