Geometric parameters, vibrational frequencies and thermochemical values of p-quinonimine (p-Q) and p-aminophenol (p-AP) were computed by ab initio calculation (HF) and density function theory (DFT) with the 6-31G(d,p) basis set. Cyclic voltammetry with a goldren electrode of p-AP solutions in phosphate buffers at pH 7.30 showed that standard electrode potential of half reaction for (p-Q) and (p-AP) is 0.728V. Standard electrode potential of half reaction for p-Q and p-AP were calculated using the sum of electronic and thermal free energies of p-Q and p-AP with normal hydrogen electrode (NHE) as a reference electrode. The results show that the theoretical standard electrode potential of half reaction for p-Q and p-AP is 0.682V at B3LYP/6-31G(d,p) level and 0.622V at HF/6-31G(d,p) level, respectively, indicating that computed standard electrode potential at B3LYP/6-31G(d,p) level are more reliable than that at HF/6-31G(d,p) level.