L-theanine is one kind of unique non-peptide amino acid, which is one of the main functional ingredients of tea. L-theanine has many physiological functions, such as antioxidative activity, regulating immune response, relaxing neural tension, and so on, which is beneficial for human or animal health. At present, the evaluation of physiological functions of L-theanine has become the research hotspot in the field of modern medicine and food science, but its mechanism remains to be further studied. L-theanine has been announced as a novel food ingredient. However, the products consisting of L-theanine as the main raw material are very rare in the domestic market, and its exploitation and utilization in the field of new drugs, health food, and new feed additives is far from adequate. The bioactivity of L-theanine, such as antioxidative activity, mechanism of down-regulating pro-inflammatory, regulating mitochondrial apoptotic pathway and mitogen-activated protein kinase (MAPK) pathway, and decreasing cells apoptosis caused by pro-apoptotic signal were summarized in this paper. This aims to provide a reference for the further research in the relevant fields and the further exploitation and utilization of L-theanine as functional ingredients.