Elevated right atrial (RA) pressure and progressive RA dilation are thought to play pivotal roles in the development of late complications after atriopulmonary connection (APC) Fontan surgery. However, no clear cut-off value for RA pressure or RA volume has been determined for stratifying the risk of developing Fontan complications. We hypothesized that RA tension, which incorporates information about both RA pressure and volume, might help predict the risk of developing complications. We retrospectively studied 51 consecutive APC Fontan patients (median postoperative period 14 years). RA tension was computed from the RA pressure and RA radius, which was calculated from RA volume measured by RA angiography. The correlation between the cardiac catheterization hemodynamic data and the complications of APC Fontan was investigated. Of the 51 patients, 28 had complications, including liver fibrosis (n = 28), arrhythmia (n = 8), protein-losing enteropathy (n = 1), and RA thrombosis (n = 1). Among the hemodynamic data, RA volume and RA tension, but not RA pressure, were significantly higher in patients with complications than in those without (P = 0.004 and P = 0.001, respectively). The cut-off level for RA tension to predict Fontan complications was 26,131 dyne/cm by receiver operating characteristic curve (area under the curve 0.79, sensitivity 71.4%, and specificity 73.9%). The present study demonstrated the significance of RA tension rather than high venous pressure for the development of Fontan complications. Amid the uncertainty about clinical outcomes, the present results, subject to further validation, may contribute to the indications for Fontan conversion.