Objectives: The Tei index is a useful Doppler index of combined systolic and diastolic function, calculated as isovolumic relaxation time plus isovolumic contraction time divided by ejection time. This study was focused on comparing the Tei index between normal fetus and fetuses of diabetic mothers (DMs). Methods: A prospective study was performed in a total of 68 fetuses. The study group underwent 2-dimensional/Doppler echocardiographic measurement of their Tei index. They included total 28 fetuses of diabetic mothers (DMs) (32–38 weeks of gestation). In this group, 18 fetuses were appropriate for gestational age (AGA), 6 fetuses were large for gestational age (LGA) and 4 fetuses were small for gestational age (SGA). Control group included total 40 fetuses (22–38 weeks of gestation). Results: In the control group, the mean Tei indices of left and right ventricles were 0.29 (0.10–0.56) and 0.31 (0.15–0.48), respectively. In the fetuses with DMs, the Tei indices of the left ventricles in AGA, and SGA fetuses were 0.310 (0.17–0.42) and 0.330 (0.28–0.38). Those of the right ventricle were 0.250 (0.10–0.44) and 0.35 (0.31–0.39). The left ventricular Tei indices in LGA fetuses with DMs were 0.490 (0.47–0.52). Those for the right ventricle were 0.530 (0.50–0.57). These values were significantly higher than the control (P < 0.001 for both ventricles). Conclusions: LGA fetuses from DMs who have an increased value of the Tei index may have abnormal myocardial performance. Tei index is a simple and useful parameter for the assessment of fetal both ventricular myocardial performance and is able to reflect fetal cardiac function.