Abstract: Micronucleolus was firstly observed during meiosis of pollen mother cells (PMCs) in a high-generation inbred line of cucumber (Cucumis sativus L.).Among the 430 PMCs observed,279 PMCs contained micronucleoli,and the percentage was 64.9%.The number of micronucleoli varied from 1 to 7 in the PMCs containing micronucleolus.Micronucleoli can be seen from leptotene through tetrad.Although the number and size varied at stages,no significant differences were observed between the meiosis Ⅰand Ⅱ.The chromosomes behavior normally and no chromosome aberrances or chromosome bridge was observed during meiosis.Three commercial varieties used to compare with the high-generation inbred line as they were planted in different seasons at different fields.The results indicated that micronucleolus only occurred in the high-gene-(ration) inbred line,and the occurrence of micronucleoli was not affected by the seasonal and environmental factors.