Antibody drug conjugates (ADCs) are becoming an increasingly common approach for drug therapy. To date, three ADC drugs have been approved by the U.S. FDA. Another 10 ADCs in the pipeline are in late-stage clinical development. From a safety and efficacy perspective, ensuring the integrity of antibody-cytotoxin conjugate during drug development and production is critical. Among the several types of conjugation chemistries, enzyme-based site-specific modification shows great potential by eliminating the interruption of the antibody-antigen interaction and provides a highly reproducible and modular conjugation system when compared to standard lysine and cysteine conjugation. Here we report the characterization of ADCs with enzymatic labeled antibody N-glycans using the Q Exactive Plus MS and Orbitrap Fusion mass spectrometers.