Abstract: The karyotype and meiosis course of pollen mother cell on Paeonia ostii‘PhoenixWhite’of cultivar group of tree peony from South Yangtse River of China have been observed and researched. Results show that the chromosome number of Paeonia ostii‘PhoenixWhite’is 10. It is liploid. Kitagawa is 2n=2x=10=6m + 2sm+2 st, which is belong to 2A type. It has not observed chromosome satellites. Extrapolated Paeonia ostii‘PhoenixWhite’with Paeonia ostii is not the different same thing, whether there ismainly disp lays in chromosome satellites. Compare with the karyotype of Paeon ia ostii and show that from Paeonia ostii to Paeonia ostii‘PhoenixWhite’evolved the course from pure to mixed and shut different grade. Meiosis is out of step. It was observed that the chromosome bridge was in anaphase Ⅰ, telophase Ⅰ and the‘diagonal’bridge, splits unequally, one pair of bridges, etc. were in anaphaseⅡ, telophaseⅡ. The unusual phenomenon in meiosis course of pollen mother cell of Paeonia ostii‘PhoenixWhite’have been analyzed from cell science of heredity level. The best time to observation of the pollen master cellmeiosis is end of February to the beginning of March in Shanghai.

Key words: Paeonia ostii, Chromosome, Karyotype, Pollen mother cell, Meiosis