The stress–strain curves of brain stem in uniaxial compression demonstrate strain rate dependency and can be characterized with three regions: initial toe region, transitional region, and high strain region, suggesting strong viscoelastic behavior. To investigate the origin of this viscoelasticity at microscale, differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectra of brain stem tissue were recorded and analyzed. The emergence of endotherm thermal domains in DSC indicates that the conformation change of biomolecules can absorb and dissipate energy, explaining the viscous behavior of the brain stem. FTIR analyses indicate that the presence of polar functional groups such as amide, phosphate, and carboxyl groups in the biomolecules takes responsibility for the viscous performance of brain stem. Ogden, Fung, and Gent models were adopted to fit the experimental data, and Ogden model is the most apt one in capturing the stiffening of the brain stem with the increasing strain rate.