Zn-doped nano-scale $TiO_2$ is a useful material in the production of both hydrogen energy and LEDs. However, scientists have raised concerns regarding the toxicity of nano-scale materials. In this study, we investigated the biological toxicity of nano-scale Zn(0.1, 0.5, and 1 mol%)-doped $TiO_2$ and pure $TiO_2$ nanoparticles using the freshwater cnidarian Hydra magnipapillata as our model organism. Zn-doped $TiO_2$ nanoparticles were prepared using a conventional hydrothermal method for the insertion of zinc into the $TiO_2$ framework. The character of Zn-doped $TiO_2$ (0.1 %, 0.5%, 1% Zn) and pure $TiO_2$ was 7-8 nm with a positive surface charge. The size was smaller than that used in previous research on the toxicity of nano-scale materials. Although, in this study, we found no significant biological toxicity in Hydra magnipapillata, there was some damage under Zn-doped $TiO_2$ and pure $TiO_2$ UV-A photocatalysis conditions. We assessed that the damage was not linked to the nanoparticles, but rather due to the photocatalytic reaction. Moreover, Zn-doped $TiO_2$ and pure $TiO_2$ nanoparticles were not shown to cause cytotoxic effects, like apoptosis and necrosis that are the major markers of toxicity in organisms exposed to nanomaterials. There was no difference in necrosis or apoptosis, as viewed with a confocal laser microscope, between the Zn-doped $TiO_2$ nanoparticle-exposed group and control groups. This study suggests that particles under 10 nm in size do not exhibit biological toxicity and that the small particle size of Zn-doped $TiO_2$ and $TiO_2$ nanoparticles decreases photo-catalytic effect.