To investigate the effect of exercise on cortisol responses and energy expenditure in obese and lean men, we measured the energy expenditure (EE) and salivary cortisol responses of six obese (age 22.83± 1.9 yr; Vo2max,35.45±3.9 ml/min/kg; BMI 18.2±1.3 kg/m2) sedentary men. The protocol was running on a treadmill for 30 minutes with 65% maximal oxygen consumption. Student t-test and one-way ANOVA with repeated measures were used to evaluate cortisol, EE, Vo2 and RER between groups during the baseline and exercise periods (P<0.05). Our results showed that during the baseline period, there were no differences between lean and obese groups in salivary cortisol. In contrast, immediately, 15 minutes and 30 minutes after the exercise, obese group had significantly higher cortisol concentration than lean group (P<0.05). In obese group, mean EE (Kcal/min or kcal/min/FFM) was significantly higher than lean group, but there were no differences in RER and VO2 between lean and obese groups (P<0.05). It can be concluded that body fat percent affects cortisol response and energy expenditure, and obese men had higher EE and cortisol levels after 30 minutes of exercise than lean men.