Background: The number of diagnostic and interventional cardiac catheterization procedures are increasing in the post-operative period of congenital heart diseases (CHD). The aim of this study was to evaluate data of patients who underwent cardiac catheterization in the early post-operative period after congenital heart surgery (CHS). Methods:We retrospectively evaluated the data of patientswho underwent cardiac catheterizationwithin 30 days after CHS. Results: Between 2010 and 2016 in our hospital, 2584 children had operations, and 2911 children underwent cardiac catheterization due to CHD. Cardiac catheterization was performed in 50 (1.9% of the surgeries) of these patients during the early post-operative period. Twenty-nine (58%) of the patients were males. The median age was 7.5 months (range: 15 days-12.5 years), and the median body weight was 6 kg (range: 3-35 kg). Twenty-eight (56%) of the patients had two-ventricle, and 22 (44%) had single ventricle physiology. The median RACHS-1 score was 3 (range: 1-6). Cardiac catheterization was performed under extracorporeal membrane oxygenation (ECMO) support in 16 of the patients. Twenty-four (48%) patients underwent diagnostic catheterization, while 26 (52%) had interventional procedures. Fifteen (30%) patients had a reoperation due to anatomic problems identified during catheterization. Major complications developed in 4 (8%) patients. There was no cases of procedural mortality due to catheterization. Conclusions: Cardiac catheterization should be performed in post-operative cardiac patients without hesitation, even under ECMO, if significant hemodynamic or clinical problems cannot be identified clearly by other noninterventional diagnostic techniques.