Mass cytometry (MC) is a bioanalytical technique that uses metal-tagged antibodies (Abs) for high-dimensional single-cell immunoassays. Currently, this technology can measure over 40 parameters simultaneously on individual cells using metal-chelating polymer (MCP) based reagents. However, MC can in principle detect up to 135 parameters with the development of new elemental mass tags. Here we report the development of a tantalum oxide nanoparticle (NP)-based mass tag for MC immunoassays. Uniform-sized amine-functionalized tantalum oxide NPs (d ∼ 5.7 nm) were synthesized via a one-pot two-step reverse microemulsion method. These amine-functionalized NPs were further modified with azide groups by reacting with azide-PEG2k succinimidyl carboxymethyl ester (NHS-PEG2k-N3) cross-linkers. The Ab-NP conjugates were prepared by reacting azide-functionalized NPs with dibenzocyclooctyne (DBCO)-functionalized primary or secondary Abs (DBCO-Ab) followed by fast protein size exclusion liquid chromatography (FPLC) purification. Three Ab-NP conjugates (TaO2-PEG2k-goat antimouse, TaO2-PEG2k-CD25, TaO2-PEG2k-CD196) were fabricated and tested in MC immunoassays. For the TaO2-PEG2k-goat antimouse conjugate, we showed that it can effectively detect abundant CD20 biomarkers on Ramos cells. For TaO2-PEG2k-CD25 and TaO2-PEG2k-CD196 conjugates, we demonstrated that these Ab-NP conjugates could be integrated into the commercial Ab staining panels for high-dimensional single-cell immune profiling of human peripheral blood mononuclear cells.