A large-scale single-crystalline ZnO hexangular prism was successfully synthesized through a simple gamma-irradiation method at room temperature and under ambient pressure. The product of a ZnO hexangular prism with a rim of 230 +/- 10 nm and a length up to 8.5 mu m was well monodisperse and showed a very strong and broad green light emission at around 577 nm. A possible formation process was also proposed.