OBJECTIVE To develop a LC-ESI-MS/MS method for olanzapine assay in plasma. METHODS Olanzapine was extracted with hexane-isoamyl alcohol. The residues were analyzed with a LC-ESI-MS/MS system (Dikma DiamonsilTM C18 column , 4.6 mm×150 mm，5 μm) with the mobile phase consisted of methanol:water with 5 mmol·L-1 ammonium formate (90∶10). A Agilent 6 410 triple quad mass spectrometer system equipped with an electrospray ionization ion-trap source was used as the detector and operated in the positive ion mode. Selected reaction monitoring(SRM) using the precursor to product ion combinations of m/z 313→256 and m/z 515.3→497.3 was performed to detect olanzapine and the internal standard, respectively. The method was used to evaluate clinical application for 36 patients after multiple oral doses of olanzapine tablets. RESULTS The average recoveries for olanzapine were 100.14%, 100.33% and 100.21%，respectively. The within-day (n=5) and between-day (n=3) precision of variation was less than 15%. The calibration curves for olanzapine had good linearity (r=0.999, n=8) within the range of 0.5～100 μg·L-1 . The limits of quantitation for olanzapine was 0.5 μg·L-1 . CONCLUSION The method provides a sensitive, accurate, precise and reliable analytical procedure for clinical monitoring of olanzapine plasma and its pharmacokinetic studies.