Objective: To describe a new sonographic marker of transposition of great arteries (TGA) during the first-trimester screening. Methods: We reviewed six cases of TGA from 2013 to 2016 in which an antenatal diagnosis of TGA at first-trimester screening (11–13 + 6 weeks of gestation) was confirmed postnatally. We specifically assessed images obtained by scanning the fetal heart in three vessels (3V) and three-vessel with trachea (3VT) views using color Doppler. The “reverse boomerang” sign was defined as a reverse curvature of right ventricle outflow tract (RVOT) at level of the 3VT view. Results: We described six cases of confirmed TGA, five singletons and one twin pregnancy, among which only two vessels and the reverse curvature of RVOT (reverse boomerang sign) was demonstrated in the first-trimester screening at level of 3VT view. Ventricular septal defects were observed in three cases, and double outlet right ventricle in one case. No other cardiac or extracardiac anomalies were identified. Termination of pregnancy was not performed in any case. Conclusion: Our series case suggests that the reverse boomerang sign may improve the early prenatal screening for TGA.