The long-term ingestion of tea catechins has been reported to reduce body fat. The aim of this study was to investigate the effect of the long-term ingestion of tea catechins on postprandial energy expenditure and dietary fat oxidation. Twelve healthy men aged 27-48 years participated in the study. The subjects consumed 350 ml of a test beverage/day that contained either a high dose of catechin (592.9 mg) or a low dose of catechin (77.7 mg) for a period of 12 weeks. Respiratory analyses were conducted before and at 4, 8, and 12 weeks during the test period, in which oxygen consumption and the excretion of ¹³CO₂ were monitored over 8 hr after a single ingestion of a test meal containing ¹³C labeled triglyceride. The excretion of ¹³CO₂ in the high dose catechin group (the HC group) was significantly increased at 4 and 12 weeks of the test period compared to that for the low dose catechin group (the LC group) (p < 0.05), and this elevation persisted at 8.9% at week 0 to 12.9% at week 12. Dietary induced thermogenesis (DIT), defined as an increased energy expenditure from the fasting baseline for 8 hr after the single ingestion of a test meal, was significantly higher in the HC group at 8 and 12 weeks compared to that in the LC group (p < 0.05) with elevation to 90.3 kcal at week 12 from 51.4 kcal at week 0. In conclusion, enhanced dietary fat oxidation and an increased DIT may play an important role in the mechanism of the anti-obesity effect of tea catechins.