The photoreaction mechanism of L-valine optical isomerization is studied by density functional theory (DFT) and ab initio molecular orbital theory. The geometric parameters of reactant, product, intermediates, and transition states, as well as the reaction energy barriers, on the reaction paths in S-0 and T-1 states are optimized at the B3LYP and MP2 levels using the 6-311++G(d, p) basis set. The equilibrium geometries of the S-1 state of valine are optimized using time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d, p) level. From the analysis of each geometric stationary point on the reaction path, the photoreaction mechanism of L-valine optical isomerization is proposed. The reaction proceeds via hydrogen transfer with the help of carbonyl O or amino N atom in the excited state. Furthermore, the effect of solvent on the isomer reaction mechanism is discussed based on the polarizable continuum model (PCM) of self-consistent reaction field theory.