Our study was aimed to evaluate the left ventricular (LV) global longitudinal systolic function of patients with diabetes mellitus (DM) using real-time three-plane speckle tracking imaging (RT-3P STI). The case group was the patients of type 2 DM with normal LV ejection fraction (EF). Then according to glycated hemoglobin (HbAlc) control level, the case group was divided into two groups, including DM1 (HbAlc<7%, n=31) and DM2 (HbAlc >or=7%, n= 37); 63 matched volunteers were chosen as control group. Conventional measurements of the LV size and function were performed. We then applied the automatic function imaging to analyze the global longitudinal systolic peak strain (GLPS) of the three apical views online, including the GLPS of apical four chamber view (GLPS-A4C), the GLPS of apical two chamber view (GLPS-A2C), the GLPS of apical long axis view (GLPS-LAX), and then generating the average GLPS (GLPS-Avg). The experimental results showed that there was no statistical difference in clinical baseline characteristics among the three groups (P>0.05). However, there were statistical differences in the ventricular wall thickness and the LV mass index of the DM1 and DM2 groups compared with that of the control (P<0. 05). No significant differences were found in LV diameters, volumes, LVEF among the three studied groups (P>0. 05). We also found significant differences in GLPS-LAX, GLPS-A2C, GLPS-Avg when we compared DM2 group with those of the DM1 group or the control (P<0. 05). We found a statistical difference in GLPS-A4C only when we compared the DM2 group with the control(P<0. 05), and no statistical difference in the strains of the three views between DM1 group and control (P>0. 05). RT-3P STI could not only accurately be used to evaluate the LV global longitudinal systolic function, but could also reflect the reduction of sub-clilical systolic function in DM patients with poor blood glucose control in the early stage.