Objective: To assess numerical sex chromosomal abnormalities of the sperms before and after radiotherapy in seminoma patients and to evaluate their reproduction risks. Methods: Three color Fluorescence in situ hybridization (FISH) was performed on sperms harvested from one seminoma patient before and after radiotherapy and before surgery. The numerical sex chromosomal abnormalities were compared. Results: The ratio of 18-X and 18-Y sperm cells among the counted 40944 ones was close to 1:1 at three time points. The incidence of chromosome aneuploidy and diploid rate (18, X, Y) significantly increased after radiotherapy when compared with that before surgery and before radiotherapy. However, no significance was observed in the aneuploid and diploid rate between pre-operation group and pre-radiotherapy (post-operation) group except for the 18-YY karyotype (0.095% vs 0.026%, p<0.05). Conclusion: Our study shows increased incidence of numerical sex chromosomal abnormalities and high risk for reproductive and genetic diseases in patients treated with radiotherapy. Three colored FISH test is recommended to evaluate the rate of numerical chromosomal abnormalities; PGD and prenatal diagnosis are advised to improve the likelihood of a successful pregnancy.