Phenmetrazine is a central nervous system stimulant currently used as an anorectic agent. The drug is abused and is reported to cause death from overdose. We describe a new derivatization method for phenmetrazine using 2,2,2-trichloroethyl chloroformate. Quantitation of urinary phenmetrazine can be easily achieved by using N-propylamphetamine as an internal standard. The phenmetrazine 2,2,2-trichloroethyl carbamate showed a molecular ion isotope cluster at m/z 351, 353, 355, and 357 (isotope effect of three chlorine atoms in the derivatized molecule) and other peaks at m/z 247, 245, 204, 114, and 70 in the electron ionization mass spectrometry, thus aiding in unambiguous identification. The underivatized phenmetrazine showed a relatively weaker molecular ion at m/z 177 and a base peak at m/z 71. The N-propylamphetamine 2,2,2-trichloroethyl carbamate (internal standard) showed a very weak molecular ion at m/z 351 and a base peak at m/z 260. Another strong characteristic peak at m/z 91 was also observed. The retention time of derivatized phenmetrazine (9.5 min) was substantially longer than the retention time of the underivatized molecule (2.5 min). Moreover, underivatized phenmetrazine showed poor peak shape (substantial tailing) while derivatized phenmetrazine had excellent chromatographic property. The within-run and between-run precisions of the assay were 1.9% and 3.2% at a urinary phenmetrazine concentration of 20 micrograms/mL. The assay was linear for urinary phenmetrazine concentration of 1 microgram/mL to 100 micrograms/mL with a detection limit of 0.5 microgram/mL.