This study was performed to improve underirable warm greenhouse environment by fog cooling system in summer season. The resultsof droplet size analysis and cooling effects for fog cooling system are summarized as follows ; 1. At the pump pressure of 70kgf/$ extrm{cm}^2$ , the mean (SMD) drop size was 22.6${mu}{ extrm}{m}$ and the maximum and minimum drop size was 45.68${mu}{ extrm}{m}$ and 1.73${mu}{ extrm}{m}$ , respectively, and almost all of the drop size was less than 40${mu}{ extrm}{m}$. 2. The temperature of fog cooling greenhouse with 60% shading was dropped more than 2$^{circ}C$ below the ambient temperature , while the greenhouse temperature without shading was 1$^{circ}C$ higher than the ambient temperature. 3. It was found that fog spraying intervals were significantly influential on cooling effect. 4. When the greenhouse was ventilated sufficiently by natural vent system, green house temperature could be maintained by 2.5$^{circ}C$ lower than the ambient temperature, while it was difficult to drop the greenhouse temperature below ambient temeperature without sufficient ventilation. 5. It was found that the temperature of experimental greenhouse could be maintained 3$^{circ}C$ to 14$^{circ}C$ lower that of control greenhouse though there were variations depending on experimental and weather conditions.