In order to reveal the cytology characters of Mikania micrantha (Asteraceae), the chromosome number and morphology of Mikania micrantha (Asteraceae), an invasive plant in China, were studied, including Shenzhen University, Shenzhen Reservoir, Shantou and Yangjiang populations from south-east Guangdong, and Pingdong and Yilan populations from Taiwan. The chromosome number 2n=36 was determined for all six populations. Their karyotypes were all characterized by having a remarkably larger, submedian centromeric chromosome pair with a secondary constriction in the middle of the long arms. The karyotype formulas were as follows: 2n= 20m+14sm+2st for both Shenzhen University and Shenzhen Reservoir populations, 2n=22m+12sm+2st for Shantou, Yangjiang and Pingdong populations, and 2n=18m+16sm+2st for Yilan population. The only previous report of chromosome number for M. micrantha from its invasive range in China, i.e. 2n=38 of Shenzhen University population, is most likely a miscount, which may have been caused by mistaking the two chromosomal fragments formed through breakage of the largest chromosome pair at secondary constrictions as two whole chromosomes. Results of our study and those previously reported from the native range of M. micrantha indicate that there indeed exists aneuploid and polyploid variation within this species, but 2n=36 (diploid based on x=18) is the only number currently known from the invading populations in China. It seems that at least in south-east Guangdong and Taiwan, China, the successful invasion of M. micrantha is not associated with polyploidy.