A sensitive,simple and rapid ultra fast liquid chromatography (UFLC)-ESI-MS/MS method was established for the simultaneous determination of 3,3,4,5,7-pentamethylquercetin (PMQ) and its possible metabolite 3,3,4,7-tetramethylquercetin (TMQ) in dog plasma using 4,5,7-trimethylapigenin (TMA) as the internal standard.The plasma sample was pretreated with acetonitrile for protein precipitation and the analytes were separated on an Ultimate XB-CN column (5μm,2.1mm× 150mm) with the mobile phase consisting of acetonitrile and water (2∶1,v/v).Detection was performed on a triple-quadrupole tandem mass spectrometer under a positive multiple reaction-monitoring mode (MRM).The mass transition ion-pair was followed as m/z 373.1-312.1 for PMQ,359.1-344.0 for TMQ and 313.1-298.1 for TMA.The validated concentration ranged from 1.272 to 3060ng/mL for PMQ and from 10.35 to 1725 ng/mL for TMQ.The lower limit of quantifications for PMQ and TMQ were 1.272 ng/mL and 10.35ng/mL respectively.The developed-method was successfully applied for the pharmacokinetic study of PMQ and its metabolite TMQ in dogs following a single oral dose.