Background: We aimed to investigate the circadian rhythm on left ventricular (LV) function and infarct size, according to the onset of ST elevation myocardial infarction (STEMI), with echocardiography in patients with first STEMI successfully revascularised with primary percutaneous coronary intervention (PCI). Methods: We conducted a retrospective analysis of 252 STEMI patients. Patients were divided into the four, six-hour periods of the day. Conventional and tissue Doppler imaging (TDI) echocardiography were performed within 48 hours after onset of chest pain. The average of peak systolic myocardial velocities (Sm) in each of the four myocardial segments and LV ejection fraction (LVEF) were calculated. Results: A negative linear correlation was shown between CK-MB levels and Sm (r= -0.209, p = 0.001). There was an oscillation between time of day and average of Sm. The lowest Sm and largest infarct size were in the period of 06:00-noon compared with period of noon-18:00 and 18:00-midnight (p = 0.029 and p = 0.031, respectively). A secondary analysis showed that both LVEF and Sm were lower in the midnight-noon group compared with the noon-midnight group (44.9 ± 7.3% versus 47.3 ± 7.9%, p = 0.018, and 7.6 ± 1.4 cm/s versus 8.2 ± 1.6 cm/s, p=0.003, respectively). Conclusions: This study has shown that there was a circadian rhythm of infarct size and LV function evaluated by echocardiography according to time of STEMI onset. The largest infarct size and poor LV function occurred in the midnight-noon period, in particular in the 06:00-noon period.