Abstract: A method for determination of 224Ra and 223Ra in seawater using a radium delayed coincidence counter (RaDeCC) is presented in this study.After pre-concentrating 224Ra and 223Ra from seawaters by adsorbing onto a column of MnO2 coated fiber,samples are measured for 224Ra and 223Ra by discrimination of decays of their daughters,220Rn-216Po and 219Rn-215Po,respectively.The RaDeCC system provides a highly effective,sensitive,and simple way to determine 224Ra and 223Ra.It is characterized by low background,and designed conveniently for field operation.Based on three years experience in measurement and data of 224Ra and 223Ra by RaDeCC,we discuss systematic uncertainties,efficiencies,background values,critical and detective limits.The results show that the system efficiencies of 224Ra and 223Ra are in the ranges of 0.47~0.59 and 0.45~0.57.The detective limits are 0.04 dpm for 224Ra and 0.01 dpm for 223Ra,with a precision of 5% for 224Ra activity and 15% for 223Ra in routine analyses.However,the uncertainties of low activities samples will increase to 10%~30% for lower than 5 dpm/100L of 224Ra and 20%~80% for lower than 0.3 dpm/100L of 223Ra.