Periodontal disease, which is majorly caused by Porphyromonas gulae, is extremely common in dogs, and is associated with serious systemic diseases such as kidney failure, heart disease and immune disorder. Ozon water is ozone-gas-dissolved water, which has antimicrobial activity by oxidizing molecules. However, both therapeutic and preventive effects of ozone water for periodontal disease in dogs are still unclear. We herein examined the antibacterial and anti-inflammatory effects of ozone water in vitro using P. gulae. Our results indicated that treatment of ozone water significantly inhibited　the growth of P. gulae until 24h post ozone water treatment as compared to purified water treatment. Ozone water also showed the anti-inflammatory effects in P. gulae-infected human gingival epithelial cells (Ca9-22). Secretions of IL-1β and TNFα were significantly suppressed by ozone water treatment compared to the treatment of purified water. As IL-1β and TNFα activate the osteoclast and finally promote bone resorption in the oral cavity, our findings imply the potential therapeutic and preventive effects of ozone water in canine periodontal disease. We are currently investigating the in vivo preventive and therapeutic effects of ozone water in dogs.