Triploid watermelon is not distributed internationally due to high seed cost and difficulties associated with developing new cultivars. Therefore, it is desirable to produce seedless watermelon from commercially acceptable diploid cultivars, using a scientific method that is chemically safe. The purpose of this study was to investigate the practicality of inducing seedlessness in two Japanese watermelon cultivars by pollinating female flowers using pollen treated with varying doses of soft-X-ray (X-rays which have a wave length of about 0.1 Å).