Abstract: An electrochemical immunosensor modified with nanocomposite was constructed to detect cytokeratin 19 fragment antigen 21-1 (CYFRA21-1) which was a lung cancer marker. Ordered mesoporous carbon CMK-3 was combined with carboxylated multi walled carbon nanotubes (CMWCNTs) to improve the electron transfer rate and achieve electrochemical signal amplification. Au nanoparticles (AuNPs) were combined furtherly to prepare an electrochemical immunosensor on a glassy carbon electrode. AuNPs could be uniformly compounded with CMK-3@CMWCNTs and combined with the CYFRA21-1 antibody through the Au－S bond, providing a large number of biological binding sites for the antibody and improving the sensitivity of the sensor, so as to realize the detection of CYFRA21-1 in serum through antigen-antibody specific reaction. Scanning electron microscope characterization showed that AuNPs were embedded in the surface and pores of CMK-3@CMWCNTs. The trends of cyclic voltammetry (CV) curves showed that the constructed electrochemical immunosensor was sensitive and effective. The linear range of the constructed electrochemical immunosensor for detection of CYFRA21-1 in serum was 0.5－1×10⁴ ng · L⁻¹, with detection limit (3S/N) of 0.2 ng · L⁻¹. The spiked recovery of CYFRA21-1 in the serum sample was in the range of 91.4%－102%, with RSDs (n=6) of the determined values less than 7.0%.