Radiotherapy is the major treatment for nasopharyngeal carcinoma, a malignant tumor of epithelial origin. In this process, a tracer with high sensitivity is pivotal for diagnostic imaging in radiotherapy. Here, we designed a novel multifunctional magnetic silver nanocomposite, Fe(3)O(4)/Ag conjugated to an epidermal growth factor receptor-specific antibody (C225), which can be potentially used for synchronous cancer therapy and diagnosis via magnetic resonance imaging. Characteristics of Fe(3)O(4)/Ag/C225 were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, ultraviolet spectra, and dynamic light scattering. The results demonstrated that Fe(3)O(4)/Ag/C225 nanoparticles were spherical and dispersed well in water. The activity of C225 was preserved ∼80% in the Fe(3)O(4)/Ag/C225 nanoparticles. Futhermore, we tested the cytotoxicity and radiosensitivity of the nanocomposite for human nasopharyngeal carcinoma cell lines (CNEs) in vitro. MTT analysis revealed that Fe(3)O(4)/Ag/C225 could inhibit the proliferation of CNEs in a dose- and time-dependent manner. The clonogenic assay indicated that Fe(3)O(4)/Ag/C225 combined with X-ray treatment could increase the sensitivity of CNEs to irradiation. In a summary, the novel multifunctional nanocomposite Fe(3)O(4)/Ag/C225 might be a potential radiosensitizer for treating malign tumors in the clinic.