Herein, a giant magnetoimpedance (GMI) sensor combined with the sandwich immunoassay was used for ultrasensitive detection of carcinoembryonic antigells (CEA).Oblong-shaped Au films were enclosed with SU-8 photoresists to form several microcavities that were treated with different concentrations of CEA.The quantification of low-concentration CEA (1 pg/mL-10 ng/mL) was achieved by using one single GMI sensor, demonstrating a good linear relationship between the GMI response and the CEA concentration.The lower detection limit for CEA was found to be as low as 1 pg/mL, which provides extreme sensitivity.We observed that high-concentration CEA (>10 ng/mL) can result in the cluster of Dynabeads, thereby reducing the target signals since the adjacent magnetic dipole fields canceled each other out.Target specificity of the GMI-based magnetic immunoassay has also been verified by testing nonspecific targets.The results obtained here suggest that the GMI sensor could serve as an ultrasensitive immunosensor for quantitative determination of biomarkers.