The number of normal seeds was significantly reduced when female flowers were pollinated with irradiated pollen. In ‘Benikodama’, normal seeds were not observed at 400–1000 Gy (Fig. 2). 200–400 empty seeds per watermelon were observed at 100–1000 Gy. In ‘Fujihikari TR’ irradiated with the 800–1000 Gy, empty seeds alone were produced (Fig. 3). The numbers of empty seeds were approximately 50–150 at 800–1000 Gy. In ‘Fujihikari TR’ fruits produce with soft-X-irradiated pollen, most empty seeds were as much small and thin (Fig. 4). However, they were very conspicuous in ‘Benikodama’. Varietal differences in number of empty seeds were observed in triploid watermelon (Kihara, 1958). In our study, ‘Benikodama’ had more empty seeds than ‘Fujihikari TR’(Table 2, Fig. 2) which indicates that the number of empty seeds is determined by the characteristics of the ovule in each cultivar.