Ursodeoxycholic acid(UDCA) is a hydrophilic gall bladder acid and has been used as a effective drug for liver disease related to in1munity. This drug inhibits secretions of IL-2, IL-4, and $IFN-{gamma}$ from T-cells and production of immunoglobulin from B-cells. Also it has been reported that UDCA inhibits production of IL-1 related to the progression of periodontal disease and activation of collagenases. The purpose of the present study was to elucidate the effects of UDCA on inhibition of periodontal disease progression using clinical, microbiological and histometrical parameters. Twelve pure bred, 16 month-old-beagle dogs were used in the study. After ligature-induced periodontal diseases were formed, experimental drugs were applied twice a day and then the results of clinical, microbiological, and histometrical parameters were measured at baselie(initiation of experiment) , 4weeks and 8weeks. The gel with UDCA(concentration 0.5%, 5% 3 dogs in each) was applied to experimental group, chlorhexidine to positive control group(3dogs) and the gel without UDCA(base) to negative control group. After induction of general anesthesia, the maxillary 2nd, 3rd premolars and 1st molar and the mandibular 2nd, 3rd, 4th premolars and 1st molar were ligated in one side selected randomly and were not ligated in the opposite side. The plaque index(PI), gingival index(GI), pocket depth(PD) and gingival crevicular fluid(GCF) volum were measured clinically. The PI and GI were measured at 3 buccal points of all experimental teeth and the GCF was measured only at the 3rd premolar in the maxilla and the 4th premolar in the mandible. In the microbiological study, the samples extracted from the 3rd premolar of the maxilla and the 4th premolar of the mandible at the center of buccal surface were analyzed aerobics, anaerobics and Streptococcus colony forming units, After clinical and microbiological examination at 8weeks, the dogs were sacrificed by carotid artery perfusion. The samples were fixed and sectioned including interproximal area, and the distance from cementoenamel junction(CEJ) to alveolar crest was measured. The results were that PI, GI and PD increased until 4 weeks and decreased at 8 weeks in three groups but the differences between all the groups were not significant. The 0.5% UDCA in non-ligated group showed remarkable decrease of GCF. The experimental group applied 5% UDCA decreased the number of aerobics and anaerobics. The distance from CEJ to alveolar crest was greater in the negative control group on both ligated and non-ligated sides, but the differences were not significant stastically.