Abstract: 25S rDNA-FISH was applied to analyze metaphase chromosome components and meiotic chromosome behavior of Chinese cabbage–head cabbage allotriploid（ACC），and pollen character and the fecundity of the allotriploid was studied. In the 25S rDNA-FISH analysis for metaphase chromosomes，Chinese cabbage–head cabbage allotriploid was verified to have 28 chromosomes and contains 9 locis of 25S rDNA，among them 4 locis mapping on chromosome 4 and 7 of head cabbage，and the other 5 locis mapping on chromosome 1–5 of Chinese cabbage. There was different degree of homology between A chromosomes and C chromosomes. At meiotic diakinesis，the chromosomes often form different association patterns，such as univalent，bivalent，and trivalent，etc. There were dissociated chromosomes at metaphaseⅠand Ⅱ，chromosome unequally segregating，lagging and losing at anaphaseⅠand anaphaseⅡ，and micronuclei forming at telophaseⅡ. These results led to that pollen regularity and pollen viability of allotriploid was much lower than that of its diploid parents. Whether this allotriploid was selfing or backcross with head cabbage，the seed setting rate was very low，only 0.01%.

Key words: Chinese cabbage, head cabbage, allotriploid, fluorescence in situ hybridization（FISH）