Fat and carbohydrates are the main substrates in the human skeletal muscle, both at rest and during exercise. Understanding the regulation of fuel selection in muscle is an important issue that needs to be clarified in clinical and sports sciences. It has been suggested that fat is the major substrate oxidised by muscle during low and moderate intensity exercise and it declines with increased exercise intensity. However, there is no specific exercise region that defines the optimal fat burning zone. During an incremental exercise test, anaerobic threshold (AT) and respiratory compensation point (RCP) reflects two important exercise intensities: onset of aerobic to anaerobic metabolic transition and onset of hyperventilation, respectively. In the present study, we have investigated the effects of exercise intensity corresponding to AT and RCP on fat and also carbohydrate utilisation. After obtaining an informed consent which was approved by the local ethics committee, nine sedentary male subjects (20.2±1.6 yr, 75.5±5.8 kg) performed an incremental exercise test (15 W/min) until the limit of tolerance using a cycle ergometer. During exercise, the ventilatory and pulmonary gas exchange indices were estimated breath-by-breath. V-slope method used to estimate AT (1) and increased VE/VCO2 and decreased PETCO2 used to estimate RCP (2). Then they participated five 30 min (or until exhaustion) stages of exercise at intensities of corresponding to 25% below AT (W1), AT (W2), RCP (W3), 25% above AT (W4) and %100 above AT (W5). The respiratory quotient (RQ) was used to evaluate substrate utilisation. Statistical analysis was done by using paired t test. Maximal exercise capacity (Wmax), AT and RCP were found to be 221±25 W, 134±21 W (60% of Wmax) and 160±24 W, (72% of Wmax) respectively. The RQ at the end of the exercise was found to be 0.925±0.05 for W1 and 0.931±0.04 for W2 (p>0.05), reflecting ~75% CHO and 25% fat utilisation. RQ decreased to 0.904±0.04 in W3 (p<0.05) reflecting ~65% CHO and 35% fat utilisation. The subjects were not able to complete 30 min of exercise duration in W4 and W5. The RQ was systematically higher than 1.00 in W4 (1.060±0.04) and W5 (1.374±0.1). The highest fat oxidation ratio (as seen a marked decrease in RQ) was observed in the work rate corresponding to the RCP. In conclusion, these findings suggest the region between AT and RCP can be used as an optimal fat burning zone. In addition, investigators should consider RCP instead of using low-moderate-high intensity criteria as determined % values of Wmax or maximal O2 uptake.