Introduction: The role of Frank-Starling mechanism is well established in systolic left ventricular (LV) function. However, it is unknown whether this law plays a role in diastolic LV function or not. Irregular RR intervals in atrial fibrillation (AF) provide the opportunity for clinical applications of theoretical mechanism. We compared the relation between RR interval and LV performances according to systolic and diastolic functions. Methods: A total of 197 patients with AF were screened. Patients with very rapid or slow ventricular rate, significant valvular stenosis or regurgitation, or acute medical conditions were excluded and 136 patients were enrolled. LV outflow peak ejection velocity (Vpe) was adjusted for the effect of pre-preceding RR interval (RR-2) using the logarithmic equation between RR-2 and Vpe. The logarithmic equation between adjusted Vpe and preceding RR interval (RR-1) was calculated in the coordinates with RR-1 from 0.6 to 1 second. From this equation, the ratio of slope to Vpe at RR-1 = 1 second (Slope/Vpe-1) was obtained. Results: Patients with normal systolic function and with a history of heart failure (HF, n=37) had a higher slope than those with normal systolic function and without a history of HF (n=72, p=0.04) and a lower slope than those with systolic dysfunction (n=27, p=0.034). Vpe-1 was not different among groups (p=0.85, Figure 1). Slope/Vpe-1 showed similar trend with slope and were more powerful to discriminate three groups than the slope. In patients with previous tachycardia induced cardiomyopathy (n=8), the management decreased slope (p=0.006) and Slope/Vpe-1 (p=0.038) without changes in Vpe-1 (p=0.57, Figure2). Conclusions: Frank-Starling mechanism plays a significant role not only in systolic function but also in diastolic function. The relation between LV pressure/volume and performance of diastolic dysfunction was between those of normal function and systolic dysfunction.