Objective To investigate the effects of CD226 knockout ( KO) on obese mice fed with high fat diet and to analyze the composition of immune cells in CD226KO obese mice for further elucidating the immunological mechanism of CD226 involved in high fat diet-induced obesity. Methods Both wild-type ( WT) and CD226KO mice were randomly divided into two groups, high-fat and normal diet groups, and fed for 14 weeks to establish the type 2 diabetes model. Immune cells in mouse spleen and peripheral blood were analyzed by flow cytometry. In in vitro experiments, NK92-MI cells were infected with pshRNA-CD226 lenti-virus to silence CD226 expression, and then qPCR was performed to detect the expression of Foxp3, TNF-αand IFN-γ at mRNA level. Results In the high-fat diet groups, CD226KO mice had lower blood glucose, serum insulin and HOMA-IR than WT mice, but higher HOMA-IS and HOMA-β. CD226KO could reduce compensatory hyperplasia of islet tissue, and significantly down-regulate the proportion of spleen NK cells in mice. The proportion of CD3-CD49b+CD25+Foxp3+regulatory NK cells (NKreg) increased significantly in CD226KO mice. CD226KO could significantly increase Foxp3 expression in NK92-MI cells and decrease the expression of TNF-α and IFN-γ. Conclusions CD226KO can alleviate insulin resistance, increase the number of islet β-cell and improve islet β-cell function in obese mice. The mechanism might be related to the up-regulation of Foxp3+ NKreg ratio.