Objective To investigate the level of CD4⁺ T cells and related mechanisms in fetal heart tissue fromfemale rats with gestational diabetes mellitus ( GDM). Methods Eighty pregnant female Sprague-Dawley rats wererandomly divided into normal control (NC) group and GDM group induced by streptozotocin. CD4⁺ cell levels were detectedin the fetal heart tissues of different groups. Western blotting was performed to detect the interleukin (IL)-17, IL-4, T-cellimmunoglobulin mucin (Tim)-3, and programmed cell death protein (PD)-1 expressions, and real-time PCR was used todetect the expression of miR-233-3p. Results The expression of CD4⁺ CD25⁺ Treg cells in the GDM group (46. 41±10. 94%) was significantly higher than in the NC group (21. 63±10. 94%) ( P <0. 01). Expression levels of IL-17, Tim-3, and PD-1 were also significantly higher in the GDM group than in the NC group, while the expression of IL-4 wassignificantly lower ( P <0. 05 or P <0. 01). The relative expression of miR-233-3p in the GDM group (0. 682±0. 104) wassignificantly lower than in the NC group (0. 466±0. 117) ( P <0. 05). Conclusions Cardiac tissue abnormalities in GDMrats may be associated with the down-regulated expression of CD4⁺ T cells, and the underlying mechanism may be associated with the down-regulation of miR-233-3p expression in the fetal rat heart tissue.