Abstract: The number and karyotype of chromosomes of Xanthium mongolicum Kitag.were studied.The results showed that the chromosome number of the species is 2n=36and its karyotype formula is K(2n)=36=26m+2m(SAT)+8sm, as same as those of Xanthium sibiricum Patrin. X.mongolicum belongs to type "1A" of Stebbins' karyotype asymmetry.Upon a comparison of the karyotype of X.mongolicum with that of X.sibiricum Patrin,it has been found that the latter belongs to type"2B", and should,therefore, be considered as more advanced than the former.