Introduction; As the survival of patients with functional single ventricle increase, the evaluation of functional status for the post-Fontan patients has become important. The cardiopulmonary exercise test (CPET) is a useful method to evaluate the postoperative functional status of the heart. We studied the postoperative functional status of the patients with functional single ventricle by CPET and compared that with functional status in a healthy group. Material and Methods; Between July 2006 and January 2009, 50 patients with functional single ventricle underwent CPET in Sejong general hospital. All of them underwent the Fontan procedure in the past, except 1 patient who underwent bidirectional carvopulmonary shunt. The mean age at Fontan procedure was 64.4 months (range 21 to 311 months) and the mean age at CPET was 13.8 years (range 6 to 37 years). The mean duration from the operation to CPET was 102.9 months (range 24 to 215 months). The NYHA functional class is ��� in 84% of patients. All data of patients with functional single ventricle were compared with that of control group (n=21). Results; The maximal VO2 in post-Fontan patients was significantly lower than that in control group (29.9 ml/kg/min vs. 35.2 ml/kg/min, p=0.013). The oxygen consumption (VO2) at anaerobic threshold (AT) was lower than control group (24.6 ml/kg/min vs. 27.59 ml/kg/min, p=0.09). The maximum Heart rate (Max HR) was lower than control group (154.1 beat/min vs. 184.9 beat/min, p=0.00). And, the METs in post-Fontan patients was lower than control group (8.23 vs. 10.1, p=0.013). The results in patients with isomerism are not significantly different compared with that in patients without isomerism. As considering ventricular type, there are also no significant differences. Conclusion; In spite of their good NYHA functional classes, the patients with functional single ventricle have decreased exercise capacity. Therefore, we suggest the close monitoring and early medical or surgical intervention Key words; Cardiopulmonary exercise test, Functional single ventricle, Fontan