Free amino acids, the key quality components of tea, are contained in higher amounts in higher grade teas. Among free amino acids in tea, the content of theanine, which is the amino acid of the highest amount, shows high correlation to the price of green tea. A capillary isotachophoresis (CITP) method was developed for the analysis of L-theanine in tea and food supplements. The optimized electrolyte system was following: 0.01 mol/l HCl + 0.02 mol/l TRIS + 0.05% HEC (leading electrolyte), 0.01 mol/L-valine + barium hydroxide to pH 10 (terminating electrolyte). Good separation of L-theanine from other components of sample was achieved within 20 min. Method characteristics, i.e., linearity (0–200 mg/l), accuracy (99 ± 2 %), intra-assay (1.5 %), quantification limit (2 mg/l), and detection limit (0.7 mg/l) were determined. Sufficient sensitivity, low labouriousness (extraction only) and low running cost are important attributes of this method. It was proved that the developed method is suitable for the routine analysis of L-theanine in green tea and food supplements containing green tea extract.