An improved procedure for precise measurement of chlorine isotope ratios in geological samples based on Cs2Cl+ion by thermal ionization mass spectrometry is reported. Using this procedure the37Cl/35Cl ratio of NIST SRM 975 is determined to be 0.318929±0.000028(1[sgrave]) with a precision of 0.009%(1 [sgrave]). The procedure has been successfully applied to study the variation of chlorine isotopic compositions in seawater and to determine atomic weight of chlorine.