Dimethylsiloxane modified methylsilsesquioxane(MSSQ) copolymers considered as low κ dielectric materials were tried to synthesized to improve the brittleness by MSSQ alone. MSSQ-dimethylsiloxane copolymers were prepared using sol-gel reaction after MTCS(methyltrichlorosilane) and DMDCS(dimethyldichlorosilane) were hydrolyzed by H2O in THF solvent. The solid phase products, soluble in various organic solvents like THF, MIBK etc., were obtained by washing acid with deionized water, and the evaporation of toluene. The local structures for MSSQ and copolymers were analyzed by (1)H-NMR, (29)Si-NMR and FT-IR. The thermal behavior and curing temperature were determined by TGA, and the loss of the copolymers having siloxane bonds were almost constant above 400℃. It was observed from (29)Si-solid NMR that D skeletons copolymerized with MSSQ after heat-treatment at 500℃ for 1 h were still remained.