In a study on mono-genomic interspecific hybridization and its improvement of crossability in genus Brassica, effects of temperature on the development of hybrid embryos were investigated in vivo and in culture in vitro of excised ovaries. In the experiment were used B. campestris L. ssp. pekinensis (Lour.) Olsson cv. Musohakusai and ssp. narinosa (Bailey) Olsson cv. Yukina as female plants. B. oleracea L. var. capitata L. cv. Chokoyayoi-kanran was used as a male plant. Pollination was done at 15°C, 20°C and 25°C. Ovaries were cultured at 15°C/10°C, 20°C/ 15°C and 25°C/20°C (day/night), respectively. The development of hybrid embryos in vivo was observed 20 days after pollination. Wide variation between female plants in the formation of hybrid embryos was observed. Development of the embryos was faster at high temperatures (20°C and 25°C) than at low temperature (15°C).