PURPOSE: To obtain seedless grapes at a low cost and with saved labor, by applying a streptomycin agent solution or a mixed agent of gibberellin with the streptomycin through dusting instead of the first treatment with the gibberellin.

CONSTITUTION: An agent containing antibiotic streptomycin and a salt thereof as active constituents (abbreviated to a streptomycin agent) or an agent containing the streptomycin agent as an effect enhancer of the plant growth regulator gibberellin is applied to the fruit cluster of grapes by any of the dusting, immersion or coating treatment to give seedless grapes by the inhibition of fertilization or growth of ovules caused in the grapes. The grapes are treated with the gibberellin after the flowering for further thickening (growth) of the grapes.

EFFECT: The treatment can be carried out in a short time with saved labor by a large-sized machine due to the inexpensive streptomycin agent without degrading the product quality caused by the hardening of the fruit pedicel. The seedless fruit ratio is high due to the mixed agent treatment with the prolonged application period, and the method is applicable to the other quality.