Diglycidyl ether bisphenol A (DGEBA) is a thermosetting polymer with excellent cross-linking properties and an irreversible network structure that forms polymer chains when chemically reacting with hardeners such as amines and anhydrides. The resulting compound has exceptional thermomechanical and structural properties. The properties of the final compound are heavily influenced by cross-linking and network structure. In the present research, molecular dynamics (MD) simulations were used to investigate the mechanical properties of chemically cross-linked DGEBA and the curing agent diethyl toluene diamine (DETDA). The MD simulation was used to perform the cross-linking, and a comprehensive study on the mechanical properties of DGEBA/DETDA was conducted. To investigate the mechanical properties, the developed model was reinforced with hexagonal boron nitride nanosheet (h-BNNS) at various weight percentages (wt.%). The results showed that the density of the neat DGEBA/DETDA increases with increasing cross-linking. It is 1.13 g/cm3 at 90% cross-linking. Almost all cross-linking densities of neat DGEBA/DETDA had higher mechanical properties. At room temperature (300 K), the elastic modulus increases from 2.58 to 2.98 GPa for cross-linking densities of 80% (EP80), 85% (EP85), and 90% (EP90). The elastic modulus of EP85 and EP90 is 3% lower and 9% higher than the experimental value (2.71 GPa), respectively. In almost all cross-linking densities, the elastic modulus of the h-BNNS reinforced DGEBA/DETDA increases with the weight percentage (wt%) of the h-BNNS. Shear and bulk modulus increase when h-BNNS is added to the DGEBA/DETDA matrix.