Abstract: The differences between the Chl-protein complexes from the old and the young leaves of cucumber were analysed comparatively by using SDS-PAGE method. The experimental results showed that there was no CPIb band-a band which belongs to PSI-in the young leaves. The 77K fluorescence emission spectra indicated that, in the young leaves, the PSI component was comparatively higher than that of the PSII, and just inverse with the old leaves. This implicated that, in the development of chloroplast, PSI forms primarily and then PSII appears. It is interesting to notice that there is a direct proportion between the F685/F735 ratio and the monomeric form /oligomeric form ratio of Chl-protein complexes.