Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) have been employed for understanding the effect of urethane and urea contents on gas separation properties of poly(urethane-urea)s (PUUs) membranes. Five membranes considered in this study are based on polytetramethylene-glycol (PTMC.), isophorone diisocyanate (IPDI), and designed ratios of 1,4-butanediamine (BDA) to 1,4-butanediol (BDO) as chain extenders. The glass transition temperature (T-g), density, fractional free volume (FFV), X-ray diffraction (XRD) pattern and radial distribution function (RDF) were calculated to examine the nanostructure characterizations of the membranes. The permeations of five gas molecules (O-2, N-2, CO2, CH4, and H2S) through the membranes were investigated. R was found that d-spacing, fractional free volume and phase separation of hard and soft segments increase with increasing urea contents in the membranes. The results indicated that the permeability of the membranes increases with increasing urea linkage in the polymer configuration. (C) 2014 Elsevier BM. All rights reserved.