Two photochromic compounds with the pyrazolonering as the photochromic functional structure, 2(1phenyl3methyl4benzoformylpyrazolone5)thiosemicarbazone(PMBPTSC) and 2(1phenyl3methyl4benzoformylpyrazolone5)smethylthiosemicarbazone(PMBPMTSC), were synthesized and characterized by elemental analysis, MS, IR spectra, NMR spectra. The crystal structure of the photocolored product of PMBPTSC was determined by single crystal XRD analysis. The results show that the photochromic phenomenon is due to the photoisomerization from enol form to keto form. Their photochromic properties were studied by powderUV reflectance spectra under the irradiation of 200—380 nm light. The firstorder rate constants of the photocoloring reaction were found to be 780×10 -3 s -1 for compound 1A and 103×10 -3 s -1 for compound 2A. Two photochromic compounds with the pyrazolonering as the photochromic functional structure, 2 (1phenyl3methyl4benzoformylpyrazolone5) thiosemicarbazone (PMBPTSC) and 2 (1phenyl3methyl4benzoformylpyrazolone5) smethylthiosemicarbazone (PMBPMTSC), were synthesized and characterized by elemental analysis, MS, IR spectra, NMR spectra. The crystal structure of the photocolored product of PMBPTSC was determined by single crystal XRD analysis. The results show that the photochromic phenomenon is due to the photoisomerization from enol form to keto form. Their photochromic properties were studied by powder UV reflectance spectra under the irradiation of 200-380 nm light. The first order rate constants of the photocoloring reaction were found to be 780 × 10 -3 s -1 for compound 1A and 103 × 10 -3 s -1 for compound 2A .