Abstract: Techniques on the determination of isotopic abundance of osrnium using negative thermal ionization mass spectrometry (N-TIMS)are discussed in this paper. Improvement on the method,for an example,loading samples together with oxidizer is proved to be useful. These methods coupled with gas bleeding can improve the ionization efficiency significantly. The stable osmium isotopic composition obtained in this experiment is concordant with those of literature. High precision data for 200 pg osmium samples is gotten,which means that the isotope compositions of trace Os (10-9~10-12)in geological samples is determinable. This technique is very important to the developement of osmium isotopic geochemistry in China.