A simple, reliable and sensitive liquid chromatography-isotope dilution mass spectrometry (LC- ID/MS) was developed and validated for quantification of olanzapine in human plasma.  Plasma samples (50 mL) were extracted with tert-butyl methyl ether and isotope-labeled internal standard (olanzapine-D3) was used.  The chromatographic separation was performed on XBridge Shield RP 18 (100 mm × 2.1 mm, 3.5 μm, Waters).  An isocratic program was used at a flow rate of 0.4 mL?min⁻¹ with mobile phase consisting of acetonitrile and ammonium buffer (pH 8).  The protonated ions of analytes were detected in positive ionization by multiple   reactions monitoring (MRM) mode.  The plasma method, with a lower limit of quantification (LLOQ) of    0.1 ng?mL⁻¹, demonstrated good linearity over a range of 0.1 − 30 ng?mL⁻¹ of olanzapine.  Specificity, linearity, accuracy, precision, recovery, matrix effect and stability were evaluated during method validation.  The validated method was successfully applied to analyzing human plasma samples in bioavailability study.