In this paper, "a" interfacial parameter in Nicolais-Narkis model is expressed by thickness "r(i)" and strength "sigma(i)" of interphase between polymer and nanoparticles as well as material properties. "a" parameter is connected to "B-1" interfacial parameter in modified Pukanszky model and the effects of "r(i)" and "sigma(i)" on "a" are explained. The negligible difference between "a" values calculated by fitting the experimental results to Nicolais-Narkis model and also, by "B-1" results confirms the accurateness of the suggested relation between "a" and "B-1" parameters. Additionally, an inverse relation is found between "a" and "B-1" parameters for nanocomposites containing spherical nanoparticles. The results demonstrate that the slight levels of "r(i)" and "sigma(i)" data give a large value of "a" which indicates the poor interfacial adhesion. (C) 2016 Elsevier Inc. All rights reserved.