Biocompatible magnetic nano-particles show great potential in bio-magnetic applications. In this work, the feasibility of magnetically labeled immunoassay using biocompatible magnetic nanoparticles is investigated. To do this, we study the synthesis technologies for magnetic nanoparticles Fe3O4, as well as the coating of various bio-probes onto the magnetic nanoparticles to detect bio-targets, such as proteins, virus, etc. Furthermore, the measurement, magnetoreduction assay, for the amount of bio-targets are developed. The results show such merits as high-sensitivity, high-specificity, high-convenience for magnetoreduction assays using magnetic nanoparticles. The whole assay platform is promisingly applied to food inspection, environment monitoring, and even human diagnosis.