In this report, we studied the fertilization and embryo development of a cross between rice and maize. The results showed that the pollen grains of maize could enter into the tissue of style of rice, and reached the embryo sac and released the sperms. Maize sperm might fuse with rice egg or polar nucleus and formed zygote or primary endosperm. But the embryo and endosperm could not be developed normally, and thus the underdeveloped hybrid seeds were formed. The reason was probably that the parental genomes could not coordinate well. This work provided a clue to solve the crossing obstacles between different species.