The early detection of low abundance anti-hepatitis C virus antibody (anti-HCV Ab) is critical for efficient diagnosis and treatment of HCV infection. In this work, a new colorimetric assay method has been proposed for the sensitive detection of anti-HCV Ab. In this method, the antibody-induced DNA strand displacement and the resulting rolling circle amplification (RCA) are integrated to generate a large amount of tandemly repeated G-quadruplex DNAzymes on the arm of the "Y"-shaped antibody. Consequently, oxidation of 3,3',5,5'-tetramethylbenzidine can be extensively catalyzed by the peroxidase-mimicking DNAzymes. Therefore, the readout signal can be greatly amplified. Further studies reveal that 0.998 pM anti-HCV Ab can be detected by this newly developed assay method. Moreover, the strategy proposed in this method can be adapted for the detection of other antibodies or bivalent targets.