Abstract: The chromosome-doubled plants of mint cultivars 68-7， 73-8 and HU-39 were cultured in vitro for multiplication， rooting and transplanting， and then their biological characters were investigated. The results showed that the multiplication rate， rooting number and transplanting survival rate of the chromosome-doubled plants were lower than that of the control plants cultivar 68-7， while the other two cultivars had approximately similar growth rate comparing with control plants. The chromosome-doubled plantlets of three cultivars had a change of leaf shape， their main stems became thicker， nodes became shorter and chlorophyll content became significantly higher. The number of chloroplasts in guard cell of stomata showed a regular pattern between chromosome-doubled and normal plants of three cultivars， which can be considered as a rapid method to assess polyploids of mint cultivars in early stage.

Key words: mint （Mentha haplocalyx Briq.）, chromosome doubling, mutation breeding