ABSTRACT: Background: This study evaluated the diagnostic efficacy of transthoracic echocardiography (TTE) and 128-slice multislice computed tomography (MSCT) angiography in congenital heart disease (CHD). Methods: Between January 2018 and August 2022, 50 patients diagnosed with CHD underwent both TTE and ECG-gated 128-MSCT. The imaging findings were compared with intraoperative observations, categorizing pathologies into cardiac, heart-great vessel, and great vessel malformations. Results: The median age of the patients was 0.45 months, and the median weight was 5 kg. Echocardiography showed a sensitivity of 89.8% and specificity of 99.12%, with an overall accuracy of 98%. MSCT had a sensitivity of 87.9%, specificity of 98.95%, and accuracy of 97.62%. There was no significant difference in diagnostic accuracy between the two modalities (χ2 = 31.796, p = 0.215), with substantial agreement (kappa = 0.901). For surgically confirmed cardiac malformations (n = 69), echocardiography had a 100% sensitivity, whereas MSCT had an 88.41% sensitivity (χ2 = 20.633, p = 0.039), with high concordance (kappa = 0.931). For heart-great vessel connection anomalies (n = 27), both modalities had an 81.48% sensitivity (χ2 = 14.115, p = 0.481), with substantial agreement (kappa = 0.887). For great vessel malformations (n = 61), the echocardiography and MSCT sensitivities were 81.97% and 88.52%, respectively, with no significant difference in performance (χ2 = 30.303, p = 0.063), indicating substantial concordance (kappa = 0.863). Conclusions: Both TTE and MSCT are highly accurate for CHD diagnosis, each with unique advantages. Their complementary use, especially where one modality is limited, enables a more comprehensive assessment, supporting clinical decision-making and surgical planning.