L–theanine is a unique non-peptide amino acid mainly originated from tea, and is beneficial for human or animal health with a variety of physiological activity, has been widely used in food additives. Its function of immunomodulation is summarized in this paper, especially the mechanism that L–theanine enhances immune function in the body by the activation of γδ T cells and the secretion of Th1 cytokines due to the inhibition of the mevalonate pathway, and by enhancing the antioxidant ability in the body due to the increase of the content of GSH, and the future prospect of further development of health products and research in the relevant fields of L–theanine is proposed.