Asymmetric Flat sheet polysulfone-polyimide (PSF-PI) blended polymeric membranes (with PI content from 5-20%) have been fabricated following phase inversion technique. The membranes have been thoroughly characterized by the measurement of porosity, mechanical properties and also by SEM, FTIR and DSC analyses. With the increase in the PI content, the mechanical properties of the membranes, like Young’s modulus, tensile strength and elongation at break, increased. SEM investigations revealed that the surfaces of fabricated blended membranes possessed adequate homogeneity and their cross-sections showed non-porous top and diminutive porous substructure. From DSC analyses it has been observed that different compositions of the blended membranes exhibited single glass transition temperatures, implying proper compatibility of the polymers. The permeance of CO2 and CH4 through the membrane increased with the increase in PI content and it gradually decreased with the increase in the feed pressure in the range of 2-10 bar. Under the present investigation, the membrane with 20% PI content exhibited the maximum selectivity for the separation of CO2 /CH4 gas mixes.