A highly sensitive immunoassay is proposed based on time-resolved inductively coupled plasma mass spectrometry with nanoparticles as tags to antibody. Instead of using traditional integral mode detection, the transient signals induced by the flash of ions in the plasma torch from the ionization of nanoparticles tagged on antibody were recorded in a time-resolved mode. Since, under certain conditions, the frequency of transient signals is directly correlated to the concentration of nanoparticle tags, the concentration of nanoparticle-tagged antibodies can be quantified by the frequency of transient signals. With the present instrument setup, gold nanoparticle (Au-NP) tags, as small as about 15 nm in diameter, can be detected. This protocol is evaluated for a competitive immunoassay and the linear range for alpha-fetoprotein is 0.016-6.8 microg/L (between 20 and 80% inhibition). The limit of quantification is 0.016 microg/L (20% inhibition, IC(20)) with a relative standard deviation of 4.2% (20% inhibition, 4 replicates) for alpha-fetoprotein. The present strategy provides a sensitive readout method for nanoparticle tags, which is quite promising for numerous applications in immunoassay, DNA hybridization, and other biological analyses.