It is now relatively well established that chenodexoycholic acid (CDCA) and ursodeoxycholic acid (UDCA), which are 7-OH epimer, are cholesterol gallstone dissolving agent. We investigated physical-chemical properties of CDCA and UDCA with respect to critical micellar concentration (CMC), micellar molecular weights (MMW) and solubilizing power of cholesterol. CMCs of CDCA and UDCA, measured by surface tension method, were 3.6 mM and 6.4 mM respectively. Light scattering technique revealed the MMW as 7700 in CDCA and 7400 in UDCA of only one molecule's difference per micelle in the aggregation number. Solubilization of cholesterol in model bile system disclosed remarkable difference. The degree of solubilized cholesterol by UDCA was smaller than that of CDCA by a factor of about 19. Addition of lecithin produced a greater increment of solubilized cholesterol in UDCA, but the solubilizing power was still larger in CDCA. It is curious and of interest that there exists much difference in the aspects of solubilizing efficiency for cholesterol in the in vitro model system, inspite of their little difference in the molecular structure, CMC and MMW between CDCA and UDCA. 7 beta-OH might affect the function of micelle against solubilization in UDCA.