To assess the effects of a proton beam on oyster mushrooms (Pleurotus ostreatus), the genetic diversity and phylogenetic relationships among strains induced by a proton beam were investigated based on a clustering analysis. According to an AFLP DNA polymorphism analysis, the induced strains were divided into four groups that coincided with the dose. When applying proton-beam radiation, the dissimilarity among the induced strains increased when increasing the dose. When using more than 400 Gy, the genetic dissimilarity of the irradiated strains was 46-58%. Thus, evaluating the induced strains using the AFLP technique was effective in revealing the mutation effect of the proton beam.