Introduction We aimed to provide reference ranges for the right ventricular outflow tract (RVOT) diameter by cardiovascular magnetic resonance (CMR) in children and compare the measurements with two-dimensional (2D) transthoracic echocardiography (TTE). Methods We measured the RVOT diameter in 49 children with normal RVOT anatomy on balanced steady-state free precession (bSSFP) CMR images in the strict transverse and sagittal views, and on 2D TTE images in parasternal short axis (PSAX) and parasternal long-axis (PLAX) views. Results Based on 63 measurements, we obtained the following mean RVOT diameters and their reference ranges, indexed to body surface area: 15.2 mm/m2 (7.0-23.3 mm/m2) in the strict transverse view, and 14.0 mm/m2 (7.8-20.2 mm/m2) in the strict sagittal view. Pearson correlation showed a very strong correlation between the CMR strict transverse view and the 2D TTE PSAX view (r=0.84; p<0.0001), and a strong correlation between the CMR strict sagittal view and the 2D TTE PLAX view (r=0.78; p<0.0001). The Bland-Altman analysis showed poor agreement between the CMR strict transverse view and the 2D TTE PSAX view (bias -3.34 mm/m2 or -16.6%), and between the CMR strict sagittal view and the 2D TTE PLAX view (bias -3.90 mm/m2 or -19.7%). Conclusions There is strong correlation but poor agreement between the measurements of the RVOT diameter in the strict transverse and sagittal views by CMR and the similar PSAX and PLAX views by 2D TTE. The static bSSFP CMR images in the strict transverse and sagittal views can’t be used to define RVOT dilatation in children