Background. Despite its importance as a prognostic indicator in cardiac morbidity and mortality, determinants of the severity of functional tricuspid regurgitation(FTR) have not been studied. Methods and Results. We prospectively enrolled 52 patients with chronic right ventricular(RV) dilation and measured right atrial area, RV area, RV fractional area change(RVFAC), tricuspid valve(TV) tethering area, TV annulus diameter and contraction, and RV systolic pressure with transthoracic echocardiography. Regurgitant orifice area of FTR(FTR-ROA) was obtained by PISA method for the quantification of FTR severity. In addition, we measured RV eccentricity index(RVEI) (Fig.A) and RV sphericity index for simple characterization of RV geometry. In multivariate analysis, end-systolic RVEI showed the best association with FTR-ROA(r=0.64, p<0.001) (Fig.B). TV tethering area and end-systolic TV annulus diameter were also correlated with FTR-ROA(r=0.43 and 0.45, respectively, both p=0.001). However, RV area, RVFAC, and RV systolic pressure did not show the relationship. With receiver operating characteristic curves, the sensitivity and specificity for predicting more than mild FTR were 81% and 75% with end-systolic RVEI >2, 81% and 62% with TV tethering area >0.65cm2, and 69% and 62% for end-systolic TV annulus diameter >3.5cm, respectively. Conclusions. The degree of FTR was largely determined by end-systolic RVEI. This finding underscores the importance of the pattern of RV geometrical change for the development and progression of FTR.