Theanine, which has an umami taste, accounts for more than half of the free amino acids in tea and is an important component that determines the flavor of tea. It is known that there is a strong correlation between theanine content and tea quality, especially for green tea. Theanine also has various physiological effects, such as relaxing effect, stress-reducing effect, premenstrual syndrome (PMS) alleviation, and so on. With the recent health food boom, theanine is getting attention day by day. Application News L529A introduced amino acid analysis by the automatic pre-column derivatization method using the co-injection mode of the i-Series. This article introduces an example of simple and quick analysis of theanine and other major amino acids, by the same method as L529A using the Nexera™ Series.