Abstract: Objective:To study the immunosuppression mechanism of Ultraviolet (UV) . Methods: In vitro,the auto lymphocyte proliferation test with LC in guinea-pig was investigated . Results: It showes that UVB significantly inhibited LC auto-stimulatory property in dose-dependent way. That enriched LC suspensions exposure to low doses of UVB(25 J/m 2) radiation does not significantly affect their auto-stimulatory capacity , the inhibition ratio of UVB irradiation is 22.4%,50% in dose of 50J/m 2 and 100J/m 2. At 200 J/m 2 it almost completely abrogated the lymphocyte proliferation.Conclusion: UVB can inhibit the function of LC directly. The mechanism of immunosuppression induced by UVB may be proposed that it may interrupt the auto-regulation of surface molecules of LC.