The complexization between folic acid and typical polyaromatic hydrocarbon (phenanthrene) was studied by using FTIR and UV spectra. The negative values of Gibbs free energies for these interactions at different temperatures showed that the process is spontaneous, in the direction of equilibrium and changed with the increasing in temperature. The enthalpy of interaction had appositive value that means the process is endothermic and the system requires input of energy. The experiments confirmed that combination was through π-π donor-accepting interaction and inducing the protonation process in folic acid upon strengthening electron accepting ability of the acid so the changing of their charge distribution and the surroundings became necessarily. In another words, this inferred that not only biotransformation process of folic acid, but also the toxicity of polyaromatic hydrocarbons could be understood.