Precise prediction of the gas permeability behavior through the mixed matrix Composite membranes (MMMs) composed of the tubular fillers using existing theoretical approaches are infrequent. This is normally due to the neglecting the interfacial characteristics of the tubular filler particles i.e. multi-walled structured carbon nanotubes (MWCNTs) and a matrix composed of polymeric material in the existing theoretical models, especially, the Kang-Jones-Nair (KJN) model and Hamilton-Crosser model (HC) which were developed for the prediction of gas permeability behavior through the mixed matrix membranes (MMMs) composed of the tubular fillers. In this work, raw- and functionalized MWCNTs filler based MMMs in polysulfone (PSF) matrix were synthesized successfully, followed by morphological analysis on matrix interfacial layers parameters. KJN model was modified by introducing pseudo-dispersed phase fillers that influenced the interfacial layer and consequently overall gas permeabilities, which was ignored in existing models. The new proposed theoretical model is able to predict the gas permeability behavior with significantly reduced average absolute relative error (AARE) of 1.26 compared to 52.43 and 42.71 for unmodified KJN and HC models, respectively. Furthermore, the mKJN model revealed that the interfacial layer thickness is a unique characteristic and is autonomous of the penetrant molecules of gas which may be influenced by the heterogeneity in the experimental conditions. The cross-sectional morphology and mKJN model revealed that the filler functionalization may lead to the improvement in filler-polymer interaction which thus reduced interfacial layer thickness. © 2020 Elsevier Ltd