When the gametocidal chromosome 3C from Aegilops triuncialis was introduced into common wheat, it was preferentially transmitted and it had severe Gc action in Chinese Spring (CS). In this study, we used the root tips of Chinese Spring and Chinese Spring carrying the gametocidal chromosome 3C to analyze the relationship between gametocidal action and DNA methylation. They were treated with the hypomethylation agent 5-azacytine (5-AC) in 100µM concentration. In the root tips of Chinese Spring we observed abnormal division at a low frequency, which less than 1%. In contrast, in Chinese Spring-Gametocidal chromosome 3C disomic addition line a mass of chromosome aberrations were observed and the major form was chromosome breakage. This suggested that during early seed development chromosome breakage induced by the gametocidal chromosome 3C was repressed by DNA methylation. In addition, after 5-AC treatment the amount of chromosome breakage might be relative to Gc action.