Primary immune thrombocytopenia (ITP) is an autoimmune heterogeneous disorder. Excessive activated CD4(+) T effector cells (Teffs) and compromised regulatory T cells (Tregs) were reported in ITP patients, yet little is known about the mechanisms. Interleukin-10 (IL-10) is an important regulatory cytokine of Tregs in inflammatory condition. It has been recently highlighted that IL-10-producing Tregs contribute to the effective controls of several autoimmune diseases. Hence this study was aimed to examine the role of IL-10 produced by Tregs in newly diagnosed ITP patients. Newly diagnosed ITP patients and healthy volunteers were enrolled to assess the numbers of peripheral Th1, Th17 cells and Tregs. CD4(+)CD25(-)Teffs and CD4(+)CD25(+)Tregs were purified. CD4(+)CD25(-)Teffs, labeled with CFSE, were cultured alone or with Tregs for 5 days, and the supernatants were then collected for IL-10 concentration test. The role of IL-10 in Tregs' inhibitory function was also determined. In ITP patients, Teffs were excessively activated, while the Tregs were numerically and functionally impaired. The percentages of IL-10(+) Tregs in Tregs' population were found elevated dramatically in ITP patients but decreased in the remitted patients. The IL-10 concentrations in the cultured supernatant were decreased in ITP patients but elevated in the remitted patients. Furthermore, the IL-10 secretion by Tregs was dramatically decreased in ITP patients. IL-10 treatment enhanced the suppression effect of Tregs toward Teffs, whereas anti-IL-10 treatment boosted the proliferation of Teffs and Th17 cells. Excessive activated Teffs and impaired Tregs play major roles in the exuberant CD4(+) T cells immune responses of ITP. The inhibitory effect of Tregs toward Teffs is largely exerted by IL-10. Insufficient secretion of IL-10 compromises the inhibitory capability of Tregs against Teffs in newly diagnosed ITP patients.