The NMR and IR spectroscopy of phenyl propenoic acid and its derivative P-Cl-phenyl propenoic acid was studied in detail. All the peaks in each spectrum were assigned. The NMR and IR spectroscopic difference between the trans-and cis-structure compound and the affection of P-Cl-substitution in benzene ring were discussed. The IR spectroscopic difference between the trans-and cis-structure is mainly due to the different strength of the intermolecular hydrogen bonding in each dimer and to the participation of the P-Cl-substituting group of the benzene ring in the π conjugate system to result in the change of NMR and IR spectra.