Objective To establish the normative data of the 2D and 3D ultrasound (US)measurements of the developing fetal thymus and comparing the 2DUS and 3DUS measurements of the fetal thymus. Methods The normal fetuses' thymus of 567 cases were assessed, and maximum transverse diameter(MTD),antero posterior diameter (APD), suprainferior diameter (SID), maximum transverse area (MTA) were measured by 2DUS,and thymic volume(TV) was measured by 3DUS. Results 2DUS,3DUS assessments of the fetal thymic MTD, APD, SID, MTA and TV were possible in 541 of 567 normal singletons. The fetal thymic 2D diameters/area and 3D volume grow with the gestational age(GA) in linear correlation. The 3D-US TV measurements and GA was significantly higher than that of any individual 2DUS measurements and GA ( P ＜0.05). Conclusions This study presents the normative data of the 2DUS and 3DUS measurements of the developing fetal thymus. 3DUS fetal thymus volume is more significantly correlated to GA than the other 2DUS measurements, which indicates 3DUS measurement of the fetal thymus is more accurate than that of 2DUS.