The mechanical instability of the Solid Electrolyte Interphase (SEI) layer in lithium ion (Li-ion) batteries causes significant side reactions resulting in Li-ion consumption and cell impedance rise by forming further SEI layers, which eventually leads to battery capacity fade and power fade. In this work, the SEI elasticity was studied both experimentally and computationally. To characterize the SEI elasticity, the SEI layer formed on a HOPG electrode in a LiPF₆ / EC+DMC electrolyte has been investigated using PF-QNM and atomistic calculations. Both experimental and computational results have shown that SEI elasticity is strongly affected by the SEI chemical composition. It was observed that the inner layer is stiffer than the outer layer. The measured Young’s moduli were mostly in the range of 0.2 to 4.5 GPa, while some values above 80 GPa were also observed. This wide variation of the observed elastic modulus was elucidated by atomistic calculations with a focus on chemical and structural analysis. The numerical analysis showed the Young’s moduli ranged from 2.4 GPa to 58.1 GPa in the order of the polymeric, organic, and amorphous inorganic components. The crystalline inorganic component (LiF) showed the highest value (135.3 GPa) among the SEI species. This quantitative observation on the elasticity of individual components of the SEI layer must be essential to analyzing the mechanical behavior of the SEI layer and to optimizing and controlling it. The findings from this work can be used in controlling the SEI layer to minimize damage to the SEI layer. The mechanical stability (adhesion, elasticity, etc.) of the SEI layer can be achieved by changing formation conditions and electrolytes (salts, solvents, and additives). For instance, the formation of an inorganic layer with an amorphous feature, and of a PEO polymeric layer rather than an EC- or DMC-based organic layer would be desirable for forming a flexible SEI layer.