Abstract: The methods of mass fractionation correction and oxygen isotope interference correction in the processing of measuring osmium isotopic compositions by negative thermal ionization mass spectrometry are described. The methods based on the practice of processing data were applicable for unspiked and spiked cases. In the former case the normalization factors of 240M/ 236M= 3.092 203 given by Finnigan MAT company and 240M/ 238M=1.553 624 recommended in the laboratory were used for mass fractionation correction, respectively. It suggested that the value suggested is independent on 187Os/ 188Os ratio of sample in study, so that it's applicable for various types of sample. In the latter case, the new normalization value with iterative algorithm method will be obtained in each iterative step and used for mass fractionation correction and oxygen isotope interference correction at the same time. The convergent results of osmium concentration and 187Os/ 188Os ratio for the sample can be calculated through five or six iterative steps based on practical data processes.