Tai Chi Chuan (TC) is a Chinese conditioning exercise and is well-known for its graceful movement. The exercise intensity of TC depends on its training style, posture and duration. Variation in training approaches result in substantial differences in exercise intensity. We have measured heart rate (HR) and oxygen uptake (VO2) simultaneously during classical Yang TC practice in 15 male subjects, their heart rate (HR) during TC practice was 58% of the heart rate reserve (HRR), and oxygen uptake (VO2) was 55% of the peak oxygen uptake (VO2peak). The level of blood lactate immediately after TC practice was 3.8mM, which reflected the level of lactate during TC approximated the onset of blood lactate accumulation (OBLA). In order to evaluate the relative exercise intensity of classical Yang TC, we measured HR responses during TC practice in 100 subjects with age of 25-80 yrs (M/F: 54/46). They were separated into three groups: young (25-44 y/o), middle-aged (45-64 y/o) and elderly (65-80 y/o). During the TC practice, the mean HR of men was 141 +/- 12, 132 +/- 9 and 120 +/- 10 bpm in the young, middle-aged and elderly groups, respectively. Meanwhile, the mean HR of women was 136 +/- 10, 126 +/- 11 and 115 +/- 12 bpm in the young, middle-aged and elderly groups, respectively. Men practiced TC with mean HR corresponding to 57.8 +/- 3.7, 56.6 +/- 3.4 and 55.1 +/- 3.1% of heart rate reserve (HRR) in the three groups; while that of women corresponding to 52.7 +/- 2.8, 51.5 +/- 2.6, and 50.3 +/- 2.9% of HRR in the three age groups. The results demonstrate that classical Yang TC is an exercise with moderate intensity, and its exercise intensity is similar across different ages in each gender.