Abstract: Objective To analyze variations of frequencies and biologic activities of lymphocytes in the uncoupling protein 1 deficient(UCP1^-/- )mice. Methods The frequencies of CD4⁺ T, CD8⁺ T, natural killer cell(NK cell), and B lymphocyte in spleen, peripheral blood, thymus or bone marrow were measured both from the wild-type(WT)and knock-out(KO)mice by flow cytometry. The CD4⁺ CD25⁺ Foxp3⁺ cell frequencies and interferon-γ(IFN-γ)production of CD4⁺ T or CD8⁺ T cells were analyzed by the flow-cytometric intracellular staining. Results Physiologically, there were no significant differences of T and NK cell frequencies in thymus and B cell frequency in bone marrow between two kinds of mice. Compared with WT mice, frequencies of total CD8⁺ T cells, activated CD8⁺ T cells(CD8⁺ NKG2D⁺ and CD8⁺CD44⁺CD62L^-)and effector CD8⁺T cells(CD8⁺ CD44⁺KLRG1⁺)decreased in KO mice. IFN-γ production by UCP1^-/- CD8⁺ T cells was also lower than that of CD8⁺ T cells in WT mice. No significant variations of CD4⁺ T cell frequency and activity in spleen and blood of the KO mice were observed. Neither NK nor B cell frequencies in spleen and blood were different between two kinds of mice. Conclusion The UCP1 deficiency may exert no effects on the development of lymphocytes, but can down regulate biologic activities of CD8⁺ T cells.