Transnational trade of 'Seedless Kishu' mandarins (Citrus kinokuni mukakukishu) would require them to be subjected to a suitable phytosanitary treatment. Irradiation is used as an effective treatment for many fruit, but the effect on quality of kishu mandarins is unknown. 'Seedless Kishu' mandarins were treated with gamma irradiation (150, 400, and 1000Gy) and stored for three weeks at 6°C and then for one week at 20°C. Irradiation at 400 and 1000Gy promoted browning of the calyx end and fungal infection. Irradiation caused immediate reductions in pulp firmness, vitamin E, individual sugars and carotenoids but increased the content of organic acids, except ascorbic acid, and phenolic compounds. The volatile profile of tested fruit was also differentially altered by irradiation. Most of these initial changes were dose dependent. 'Seedless Kishu' mandarins are significantly sensitive to irradiation and are not suitable for treatment at the studied doses.