The purpose of this study was comparing the fat and carbohydrate oxidation at differentintensities of exercise between professional and Elite Amateur cyclists. Methods: Forty-twoprofessional cyclists (27.3± 4.2 yr) and 31 Elite Amateur cyclists (23.7±3.6yr) performed anincremental exercise test on a bicycle ergometer with 50 Watt increments every third minuteuntil volitional exhaustion. Heart rate; the average volume of oxygen consumption; productioncarbon dioxide and respiratory exchange ratio were measured during the rest time and the last 3minutes of the exercise with working loads of 200;300;400 Watt. Results: The results showedthat there was no significant difference in RER between two groups at rest time and intensity of200w; but at intensities of 300w and 400w; RER was significantly lower in professional cyclists(P=0.01). There was no significant difference between professional and elite amateur cyclistsregarding the rate of fat and carbohydrate oxidation and the percentage of fat and CHOoxidation at different intensities of exercise. Conclusion: The findings of present studysuggested that there is a little difference in fat and carbohydrate oxidation between professionaland elite amateur cyclists; but this very little difference and the little carbohydrate saving inprofessional cyclists leads to their superior performance.