A rapid and sensitive assay based on high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) was developed for the determination of ranitidine(RAN) in human plasma with codeine as internal standard(IS). After protein precipitation with acetonitrile, the analyte and IS were separated on a Zorbax SB-Aq C18 column(150 mm×4.6 mm i. d., 5 μm) eluted with a mobile phase consisting of methanol/acetonitrile/10 mmol/L ammonium acetate containing 1% formic acid(pH=2.4)(volume ratio 12.5:12.5:75) at a flow rate of 1.0 mL/min. Detection was performed by electrospray ionization in the positive ion mode followed by the multiple reaction monito- ring(MRM) of the transitions of RAN at m/z 315.1→176.3 and of IS at m/z 300.1→165.1. The method was linear over a concentration range of 1―1000 ng/mL(r=0.9991) with a lower limit of quantitation(LLOQ) of 1 ng/mL and a limit of detection(LOD) of 0.3 ng/mL. Accuracy as relative error was from –0.01% to –1.7% and intra-day and inter-day precisions as relative standard deviation were ≤8.9% and ≤5.5%, respectively. The method was successfully applied to a pharmacokinetic study of ranitidine, getting a single oral dose(160 mg) to healthy volunteers.