In this study, the Ji-developed model for modeling the young modulus of nanocomposites was suggested to predict interphase thickness (Y) and modulus (Ei). The resulting model was evaluated for accuracy by comparing it to the experimental young moduli of various samples. Additionally, the effects of using a compatibilizer or modified nanocellulose on the interphase thickness and modulus of nanocellulose composites are investigated. Those samples exhibiting the highest Y and Ei parameters are TPS/NFC, PA6/NCC, and PVA/NCC, which can form hydrogen bonds with nanocellulose hydroxyl groups due to their ability to form hydrogen bonds. When compatibilizers are applied to nanocomposites or modified nanocellulose is applied in the polymer matrix, interphase parameters increase. In PLA/NFC composites, MA coupling agents were used to increase Y by 4000%, while Ei increased by 1045%. Moreover, the Y and Ei of CNCs modified with aminopropyl triethoxysilane (APS) in polyamide-6/NCC increased from 0.35 to 0.69 (97.14%) and 35.26 to 58.7 (66.47%). The Ji model was also investigated with and without considering the interphase thickness. As a result, the model became better fitted to experimental young modulus results by considering the interphase parameter in nanocomposites.