The popular bio-marker for colon cancer is carcinoembryonic antigen (CEA). By conjugating anti-CEA onto magnetic nanoparticles, CEA can be specially labeled and detected by measuring magnetic signals via immunomagnetic reduction (IMR). The low detection limit and detection range of IMR on CEA are investigated. The results are compared with those by using the existing assay, such as enzyme-linked immunosorbent assay (ELISA). It is evidenced that IMR has sensitivity much higher than that of ELISA. The low detection limit is below the normal level of CEA concentration of clinic practice and is suitable for early-stage in-vitro diagnosis for colon cancer. Furthermore, the dynamic range of detection for the CEA concentration using IMR extends well above the threshold of high risk level of colorectal carcinoma in current diagnosing practice. Therefore, IMR is also suitable in other stages diagnosis for colon cancer development.