Mertansine, a thiol-containing maytansinoid, is a tubulin inhibitor used as the cytotoxic component of antibody-drug conjugates for the treatment of cancer. Liquid chromatography-tandem mass spectrometry was described for the determination of mertansine in rat plasma. 50-µL rat plasma sample was pretreated with 25 µL of 20 mM tris-(2-carboxyethyl)-phosphine, a reducing reagent, and further vortex-mixing with 50 µL of 50 mM N-ethylmaleimide for 3 min resulted in the alkylation of thiol group in mertansine. Alkylation reaction was stopped by addition of 100 µL of sildenafil in acetonitrile (200 ng/mL), and following centrifugation, aliquot of the supernatant was analyzed by the selected reaction monitoring mode. The standard curve was linear over the range of 1–1000 ng/mL in rat plasma with the lower limit of quantification level at 1 ng/mL. The intra- and inter-day accuracies and coefficient variations for mertansine at four quality control concentrations were 96.7–113.1% and 2.6–15.0%, respectively. Using this method, the pharmacokinetics of mertansine were evaluated after intravenous administration of mertansine at doses of 0.2, 0.5, and 1 mg/kg to female Sprague Dawley rats.