An ultra high performance liquid chromatography-tandem mass spectrometric(UPLC-MS/MS) method was established for the determination of two tetrodotoxins(TTXs),eg TTX and 4,9 anhTTX,in puffer fish and nassarius with a direct dilution technique.Samples were extracted with acetic acid-methanol(1∶99,by volume).After centrifuged and diluted by 80% methanol solution，the extract was separated on an ACQUITY BEH Amide(100 mm×2.1 mm，1.7 μm) column by gradient elution,with 0.1% formic acid solution containing 05 mmol/L ammonium formate-0.1% formic acid acetonitrile solution as mobile phases,analyzed in ESI+ mode under multiple reaction monitoring(MRM) mode and quantified by external standard method.There were good linear relationships in the range of 0.5-100 μg/L for TTX and 0.5-50 μg/L for 4,9 anhTTX,with their correlation coefficients(r2) larger than 0.99.The limits of detection(LOD) and limits of quantitation(LOQ) were in the ranges of 100-150 μg/kg and 300-450 μg/kg,respectively.Recoveries at three spiked levels ranged from 75.4% to 96.8%,with intra-day relative standard deviations(RSD) and inter-day RSD of 2.3%-9.5% and 5.3%-9.2%,respectively.The method was simple and high sensitive,and was suitable for the determination of two TTXs in puffer fish and nassarius.