Abstract: This study investigated the effects of IUGR (Intrauterine growth retardation) during late pregnancy on the fetal thymus and the T cell lymphocyte development of Mongolia ovine. 42 Mongolia ewes mated at a synchronized estrus were selected and among them 6 ewes were slaughtered at the beginning of the experiment and the remaining 36 animals were allocated to three different groups: restricted group1 (RG1, 0.175 MJ·kgW^-0.75·d^-1, n=14), restricted group2 (RG2, 0.33 MJ·kgW^-0.75·d^-1, n=12) and control group (CG, ad libitum, 0.67 MJ·kgW^-0.75·d^-1, n=10). At 140 d of gestation, 6 ewes in each group were slaughtered and their fetuses were removed. The results showed that the fetal weight in both RG1 and RG2 groups were significantly reduced(P<0.01) by maternal under-nutrition during late pregnancy, which led to fetal thymus weight, thymus cortical thickness, thymus DNA contents in RG1(P<0.01) and RG2(P<0.05) groups, and thymus cortical/medullary ratio, protein contents in RG1(P<0.01) were decreased compared to that in CG group. The activities of SOD(P<0.05) and the contents of T-AOC(P<0.01) of fetal thymus in RG1 group were decreased, while the activities of GSH-PX and the contents of MDA were increased compared to CG group（P<0.05）. Additionally, the CD4⁺CD8⁺ T lymphocyte subsets of fetal thymus were lower in both RG2 and RG1 groups than that in CG group(P<0.05). These results indicate that the fetal thymus and T lymphocyte are affected seriously by IUGR during late pregnancy, which may influence fetal immunity capability and postnatal health.