Abstract: A high precision method has been developed for analyzing uranium and thorium in seawater by MCHCP-MS in this study.The optimal raio of NH₃·H₂O with acidified seawater of pH=2(0.14 mol/dm³)is quantifiedfor Mg(OH)₂coprecipitation protocol.The instrument and solvent background,abundance sensitivity,memoryeffects,mass fractionation and IC gains are properly corrected using CRM-45U standard.Overal,the accuracy ofu/"U andU/U are better than 0.05%and 0.08%with precisions beter than 0.12%and0.10%,respec-tively,in about 3 month measurements of CRM145 Ustandard.This method has been successfully applied to ana-lyze triplicate suface seawater samples from the Taiwan Strait.The average concentrations ofU and²Th are 3.05±0.05μg/kg and(1.66±0.04)×10³pg/kg,respetively,while the average ratios of²"u/²U and2Th/2°Th are(6.28±0.11)×10-⁵and(1.95±0.01)×10⁹,respectively.The standard deviations are lessthan 3%for Uisotopes and 2%for Th isotopes.The results are consistent with hydrological characteristics and iso-topic provenance in the study area,which support the reliability of the method.