Abstract: Summary It is hard to obtain seeds from Pinus koraiensis crossed with P. tabulaeformis. According to the cytological investigations on both pines, the developments of each macrospore and microspore, the fertilization, the number of chromosomes and the causes of failure to get seeds, indicate that the incompatibility exists in these two species of pines. 1.The interval between pollination and fertilization of P. koraiensis is almost exactly 12 months, but that of P. tabulaeformis is 13 months. After the pollination has taken place, both the pollens stay at the pollen chamber of the tip of nucellus and will germinate until the average temperature is above 22 pollinations take place while the female gametophyte is still in a free nuclear stage. The mitosis of the central cell and the body cell of the pollen tube are at the some time, which in P. koraiensis is 102 days before fertilization, and which in P. tabulaeformis is 7days. 2.There is a big difference between the number of chromosomes of P. koraiensis and that of P. tabulaeformis. Chromosomes in the microspore mother cell of P. koraiensis is n=6, those of P. tabulaeformis is n=12; chromosomes in the endosperm cell of P. koraiensis is n=6, 70, those of P. tabulaeformis is n=12, n=14. As for the number of chromosomes the crossing of P. koraiensis with P. tabulaeformis is rather difficult. 3.The main cause of the incompatibility between P. koraiensis and P. tabulaeformis is the pollen of P. tabulaeformis showing poor or no germination in the nucellus of P. koraiensis. The ovule of P. tabulaeformis stopped its development at the free nuclear stage and is similar to the development of the ovule of P. koraiensis which is not fertilized.