Abstract: Objective To evaluate the impact of HIV infection on the reaction intensity of HCV antibody to ELISA reagents among intravenous drug users, providing evidence for the development and revision of the diagnostic workup for HIV and HCV co-infection. Methods The intravenous drug users were divided into two groups as whether or not they were infected with HIV. The distribution of the reaction intensity to KHB and Ortho reagents (S/CO1) was compared between the two groups. The analysis was performed using SPSS 15.0. Results Statistical differences were present in the S/CO constituent of the reaction intensity using different reagents (P＜0.05); higher proportion of low S/CO was observed in the HIV-positive group than in the HIV-negative group (P＜0.05). Statistical significant differences were also remarkable in the consistency between the KHB and Ortho reagents when imposed to low S/CO samples (P＜0.05). Conclusion The status of HIV infection exerts certain influence upon the S/CO of HCV antibody subject to both the KHB and Ortho ELISA reagents. Uncertainty or false negative reaction of HCV antibody may be present in people infected with HIV in ELISA detection, and potential false-positives may obscure weak positive samples.