A highly sensitive protein detection method based on a novel enzyme-labeled gold nanoparticle (AuNP) probe has been developed. In this method, we firstly prepared the enzyme-labeled AuNP probe by coating AuNP with antibody, single-stranded DNA (ssDNA), and horseradish peroxidase (HRP). Magnetic microparticle (MMP) functionalized with another antibody was used as capture probe. Then, target protein was sandwiched by the enzyme-labeled AuNP probe and the capture probe through immunoreaction. The target immunoreaction event could be sensitively transduced via the enzymatically amplified optical signal. By using this strategy, carcinoembryonic antigen (CEA), as a model protein, was detected with high sensitivity and good specificity. The detection limit for this approach was 12 ng L⁻¹, which was approximately 130-fold more sensitive than the conventional enzyme-linked immunosorbent assay (ELISA). The practical application of the proposed immunoassay was carried out for determination of CEA in serum samples. The demonstrated capability of the proposed method shows potentially applications for early diagnoses of diseases.