Abstract: It is well known that the chlorophyll A (Chla) is a primary pigment involved in the photosynthesis process of plants. The molecule Chla consists of porphyrin and phytol, with the magnesium ions being complex at the center of the porphyrin. The work presented in this paper was to investigate the functional group in the Chla during the photosynthesis process using molecule modification. The excitation and fluorescence spectra were taken with leaves and the relevant molecule samples prepared in the laboratory, including Chla, pheophytin A and pheophorbide A (Pha). The spectra of porphyrin were also detected from the protoporphyrin water solution. It was found that all the spectra taken from those samples were similar with a strong emission band in red.　It suggests that the functional group of the Chla in the photosynthesis process is porphyrin.