Amphiphilic block copolymer poly(1,1-diethylsilabutane)-block-poly(2-hydroxyethyl methacrylate) (poly(SB-b-HEMA)) was synthesized by addition of 2-(tert-butyldimethylsiloxy)ethyl methacrylate to a living poly(1,1-diethylsilabutane) end-capped with 1,1-diphenylethylene and subsequent hydrolysis of silyl-protecting groups. These copolymers formed spherical micelles in methanol. Small-angle X-ray scattering (SAXS) measurements revealed that the aggregation number of the micelle strongly depends on the polymer composition, while total micellar size is almost independent of the degree of polymerization of HEMA. SAXS measurements were also performed for the polymer with the highest degree of polymerization of SE in methanol/toluene mixed solvents. The polymer in the solution was found to have different morphology depending on the mixing ratio of the solvent; i.e., micelle-unimer-reversed micelle transition with increasing toluene content was clearly observed.