Abstract: A novel nematic liquid crystal compound containing a cinnamoyl moiety (PCPC) and a typically cholesteric liquid crystal cholesteryl cinnamate (CC) were synthesized to explore the mechanism ofcinnamoyl compounds, and the chemical structures of photodimerizationwere confirmed by Fourier transform infrared spectroscopy and 1H nuclear magnetic reso-nance spectral analysis. The photoreaction behaviors of these two cinnamoyl compounds in mesomorphic state and solution were investigated, UV-Vis spectral analysis was used toanalyze the photoproduct. The results show that the photochemistry of PCPC in nematic state involves both photodimerization and photoisomerization, while CC shows a complex reaction which can be divided into three parts, and this has enabled us to present new data and interpretations regarding the 2+2 photocycloaddition reaction. Additionally, the re-sults of UV-Vis spectral analysis in solutions strongly suggest that UV-Vis spectral analysis can be used to study the kinetic behaviors of cinnamoyl moiety photoreaction.