Goldfish embryos at five different developmental stages, from fertilized eggs to heart beating stage, were irradiated by UV rays, and hatching rate, darkly pigmented eye rate and abnormal embryo rate of the irradiated embryos were investigated. Being subjected to very low amount (≤3min.) of the UV irradiation, the embryos earlier than gastrula stage showed hormesis. However, the embryos at gastrula or heart beating stage were very sensitive to UV irradiation, showing just damage effect, which was very strong even at very low amount of the UV irradiation. The results also showed that development of the gastrula embryos irradiated by the UV rays stopped before darkly pigmengted eye stage, whereas embryos irradiated at heart beating stage by the UV rays could develop to the darkly pigmented eye stage, though they could not hatch out.