Due to unique optical properties, facile bioconjugation, and biocompatibility, colloidal gold nanoparticles have been widely used for nuclear targeting, gene and drug delivery, and biomedical diagnosis and therapeutics. Various peptides, proteins, and polymers, etc. have been utilized to functionalize gold nanoparticles. Especially antibodies-AuNPs conjugates have drawn more and more attractions due to their specific affinities to the target antigens. However, to our best knowledge, immunoglobulin M (IgM) has not been applied for AuNPs modification so far. In this work, we present a simple modification process to immobilize IgM on the surface of gold nanoparticles. Dark field microscopy, light scattering, and colorimetric assays were conducted to monitor the conjugation process. The properties of AuNPs before and after IgM modification were further studied. The results demonstrated that the stability of AuNPs was improved upon IgM conjugation. This study may open a new way to apply IgM conjugated AuNPs for biomedical potential applications.