Activity of PAL (phenylalanine ammonialyase) could be detected an hour after the imbibition of the robinia seed, and it was unceasingly heightened along with the growing process of the juvenile seedling. In the juvenile seedling of robinia 30 days after its germination, the activity of PAL varied greatly with the following order of different tissues stem>leaf>root. And the activity of PAL coincided with the increasing tendency of the numbers of vessel element in the plumular axis or juvenile stem of robinia and its degree of lignification as well. Therefore, it might play an active role in enhancing the resistance of seedling against diseases, insects and unfavourable environment.