Abstract Background: diastolic dysfunction has been reported as a cardiovascular effect of hyperthyroidism, and is also supposed to be a cause of heart failure manifestations in the patients with preserved ejection fraction (EF). Methods: for evaluation of diastolic function in hyperthyroid patients and also diastolic Function during performing exercise stress echocardiography, we conducted this study on 26 newly diagnosed hyperthyroid patients compared to 26 healthy subjects as control group. Diastolic function of the patients at rest was assessed using these parameters as follows: left atrium volume index, tricuspid regurgitation (TR) velocity, mitral inflow early diastolic flow (E wave), tissue doppler of mitral annular velocity (e’), E/e’ ratio, and isovolumic relaxation time (IVRT); and during exercise stress echocardiography, we judged diastolic response by measuring E/e’ ratio and TR velocity at the peak of stress in terms of the American Society of Echocardiography guideline. Results: the mean age of the patients was 39.77±12.0 years old and 53.8 % of them were female. Cardiovascular symptoms among the patients were the followings: palpitation (46.2%), exertional dyspnea (19.2%), and atypical chest pain (7.7%). All the patients had normal diastolic function at the time of resting and there was no significant difference in diastolic parameters between the two groups except for IVRT, which was lower in hyperthyroid patients. Interestingly, no patients developed diastolic dysfunction during exercise stress echocardiography. Conclusion: our finding did not support Hyperthyroidsm associated diastolic dysfunction as a cause of exertional intolerance and dyspnea in the patients with preserved EF.