Based on the differences of pollen grain shape among clones in Populus euphratica and Populus tomentosa,we studied the accuracy of estimate in percentage of 2n pollen determined under different standards. The average diameter was 32. 4 μm for P. tomentosa,which was slightly larger than that of P. euphratica( 29. 7 μm). Clonal effect was significant for diameter of pollen grains. The theoretical percentage of 2n pollen could be estimated based on the rates of tetrad,triad and dyad at tetrad stage. However,th...