Abstract: Karyotype analysis was performed for Leonurus japonicus Houtt with the localization of 45S rDNA loci by using the rDNA from Lycopersicon esculentum as probe. The meiotic observations showed that the chromosomes were paired as 10 bivalents in its pollen mother cells at diakinesis，indicating that the plant has 2n = 20. The meiotic divisions belong to the simultaneous type and the stainability of the pollen grains was 97.10%. The signals from 45S rDNA probe were detected at the terminal parts of the short arms of second pair of chromosomes and central parts of the third pair of chromosomes. The karyotype formula was 2n = 2x = 20 = 2m + 16sm（2SAT）+ 2st，with the constitution of relative length being 2n = 2L + 8M2 + 10M1. The ratio of the longest / the shortest chromosomes（L/S）was 1.58，and the asymmetry index was 69.39. The chromosome relative length ranged from 8.05% to 12.73%，and karyotype type belonging to“3A”．The idiogram was established for the species.