Abstract: AIM: To study the expression of inducible costimulator (ICOS) on human peripheral blood T cells (HPBTC) and the intervening effect of oxidized low-density lipoprotein (ox-LDL). METHODS: HPBTCs were cultured in vitro and incubated with 100 mg/L ox-LDL for 24 h. ICOS expression levels were determined by fluorescence, reverse transcription PCR (RT-PCR) and Western blotting, respectively. RESULTS: Fluorescence microscopy showed that fluorescence signals of positive cells expressed by ICOS were distributed on cell membrane in a spot-style. RT-PCR of mRNA derived from the cultured HPBTC showed products of the expected size (368 bp). Western blotting of protein verified HPBTC expression of ICOS as a cell-associated protein with a molecular mass of 55 kDa. ox-LDL significantly increased in the expression of ICOS. Similar effects were seen in both expressions of mRNA and protein (P<0.05). CONCLUSION: ICOS can express on HPBTC and ox-LDL upregulates the expression of ICOS, which may be one of the immunomechanisms of atherosclerosis.