Abstract: 10-hydroxycamptothecin，camptothecin analogue，is an antitumor agent that targets the nuclear enzyme topoisomerase I. 10-hydroxycamptothecin is injected by sodium salt form in clinic, and myelosuppression is the major toxicity. To enhance water solubility and reduce the toxicity, lipid nanoparticle which is water-soluble was designed. The quantitative analyses of liposomal and total 10-hydroxycamptothecin in dog plasma were developed and validated by liquid chromatographic-tandem mass spectrometry(LC-MS/MS). Two preparation procedures were developed to separate liposomal 10-hydroxycamptothecin, one was solid phase extraction, the other was liquid-liquid extraction. The analyte and internal standand(camptothecin) were separated on a Zorbax SB-C₁₈ column using the mobile phase consisting of V(acetonitrile):V(water):V(formic acid)＝70:30:0.2. Electropray ionization source of MS was applied and operated in positive ion mode. The peak area of the m/z 365→321 transition of 10-hydroxycamptothecin and that of m/z 349→305 transition of the IS were measured. The linear calibration curve for liposmal 10-hydroxycamptothecin is obtained in the concentration range of 1.00- 1 000μg L^－1, and that for total 10-hydroxycamptothecin is obtained in the concentration range of 1.00- 2 000μg L－1. The recoveries of solid phase extraction and liquid-liquid extraction methods are 48.1%-52.4% and 79.6%-83.0%, respectively. This validated LC-MS/MS assay is successfully applied to pharmacokinetic study of 10-hydroxycamptothecin loaded lipid nanoparticle in dogs after administration single dosages of 0.5, 1, 2 mg kg^－1 and multiple dosage of 1.0 mg kg^－1 d^－1 10-hydroxycamptothecin lipid nanoparticle.