We found that diploid seedless watermelon can be produced by pollination with partially functional pollen which was irradiated with soft X-rays at the doses of 800 to 1000 Gy. The diploid seedless fruit was almost similar to the normal fruit in size, shape, color, rind thickness, sugar content, days to maturity from pollination and rate of fruit set. The number of empty seeds in the diploid seedless fruit varied among the cultivars used in this study. No correlation was found between the number of empty seeds in seedless fruit and the number of normal seeds in normal fruit. The pollen tube of the pollen irradiated with soft X-rays penetrated normally into the synergid and sperm cells were discharged. Subsequently, the egg nucleus and sperm nucleus became attached to each other in the egg cell and a globular embryo was formed. However, the embryo failed to differentiate to organ tissues and degenerated.