Abstract: Incompatible barriers were studied under artificial pollination with light microscopy, epifluorescence microscopy and scanning electron microscopy in intersectional crosses of Populus euphratica Oliv. And P. simonii Carr. The results are as follows.1. Populus euphratica Oliv.×P. simonii Carr.A considerable amount of pollen grains of Populus simonii Carr. May germinate rapidly on the stigma of P. euphratica Oliv. And penetrate into the stigmatic tissue easily. Pollen tubes of those appear in twisting, coiling, creeping and bursting fashions are in rare cases. Some of the tubes can grow normally through the style and enter into the ovarian cavity, while others cease its growth due to the deposition of the pollen tips. The megasporogenesis stagnate for 6—7 days as the pollen tubes come around the ovules. Having degenerated before the maturation of the embryo sac, the pollen tubes lost its viability to enter into the embryo sac. Thus no fertilization proceeds.2. Populus simonii Carr.×P. euphratica Oliv.The pollen grains of P. euphratica Oliv. May germinatc on the stigma of P. simonii Carr. Only a few pollen tubes penetrate a straight path to the stigma. While a lot of pollen tubes appear to have a twisting, coiling, bulging and creeping fashions. Some of them are inhibited from further growth and some do penetrate into the stigma surface finally. Callose deposits occur in the stigmatic pailla. However, pollen tubes that penetrate through the stigma may grow normally into the style and then enter the ovary cavity with two well-developed sperms. Fertilization takes place 4—7 days after pollination. A considerable amount of embryos abort at various developmental stages, especially at the proembryo stage. A small quantity of them may develop to its full size and run to mature seeds.