The significant reinforcement of polymer nanocomposites containing spherical nanoparticles which exceeds the predictions of classical models is commonly attributed to the formation of an interphase between polymer and nanoparticles. In this article, the Christensen–Lo model for Young's modulus of polymer composites is developed by an “a” interphase parameter. Firstly, the “a” interphase parameter is approximately expressed by nanofiller and interphase properties. Secondly, the Ji model suggested for three phase systems (matrix, filler and interphase) is used to calculate the interphase thickness and modulus. The experimental data of several samples are applied to the developed Christensen–Lo and Ji models to calculate the “a” parameter and interphase properties. At the final step, the “a” parameter is accurately defined as a function of the specific surface area (A_c), density (ρ_f), modulus (E_f) and radius (r) of nanofiller as well as the thickness (r_i) and modulus (E_i) of interphase using the obtained calculations. All the predictions by the developed Christensen–Lo model demonstrate a right consistency with the experimental results, which validate the present analysis. The “a” values are obtained from 0.8 to 19 for several samples showing the different levels of interphase properties in the reported samples. Also, the calculations reveal an inverse correlation between “E_i” and “r_i” in the polymer nanocomposites containing spherical nanoparticles.