This study presents a rule of mixtures in which Young’s modulus is evaluated using the degree of crystallinity and the degree of orientation as internal variables, thereby enabling direct prediction of the Young’s modulus of thermoplastic resins from crystallization structural information. To identify the parameters for this rule, a computational framework was developed using molecular dynamics (MD) simulations, combining the United Atom (UA) model and All-Atom (AA) model. This framework facilitates the reproduction of crystal structures and the quantitative evaluation of Young's modulus. First, a polyethylene (PE) melt consisting of 300 chains, each with 1000 CH2 units, was created using the UA model. This melt was then uniaxially stretched, introducing localized molecular orientations that act as nuclei for crystallization. A 200 ns relaxation calculation under isothermal-isobaric conditions subsequently reproduced the crystalline structure. The degree of crystallinity and degree of orientation were evaluated from the resulting structure. Next, the structure constructed using the UA model was mapped to the AA model, and Young’s modulus was evaluated through uniaxial tensile simulations. The Young’s modulus obtained from the AA model was in good agreement with experimental data in terms of order of magnitude. Finally, using data from MD simulations, a rule of mixtures was formulated to calculate Young’s modulus with degree of crystallization and degree of orientation as internal variables. The proposed rule of mixtures shows good agreement with experimental results for the Young's modulus of isotropic crystalline polyethylene. This rule of mixtures enables direct prediction of Young's modulus based on crystallization structure information, reducing the experimental trial-and-error traditionally required. This approach allows for the efficient optimization of processing conditions and material performance.