The influence of cholesterol with various molar fractions on the structure of disialoganglioside (GD(1a))/cholesterol mixed micelle was investigated by using synchrotron radiation small-angle X-ray scattering (SR-SAXS). The radius of gyration (Rg) of the micelles, estimated from the obtained data, was increased with increasing cholesterol content. A multi-shell model analysis for micelles, that was successfully applied to the structure estimation of GD(1) micelles by Hirai et al. (1996), revealed that the cholesterol-dependent increase of Rg of the micelles is due to an elongation of its prolate spherocylindrical structure. In the present analysis, we assumed the micelle of the ganglioside/cholesterol mixture as a triple-shell structure taking into account the high electron density of the steroid ring of cholesterol that is inserted in the hydrophobic region of the micelle. The examined molar ratios of [ganglioside]/[cholesterol] were 1:0, 1:0.05, 1:0.1, 1:0.2, 1:0.4, 1:0.7 and 1:1. The ganglioside concentration for all samples was constant at 0.5 % w/v.