In the case that a self-organized diblock copolymer pattern is applied as an etching mask, one of the polymer components is required to have a high etching durability similar to that of silicon oxide. One suitable material for the purpose is a mixture of polystyrene-b-polyethyleneoxide (PS-PEO) and silsesquioxane (SSQ). After phase separation, the SSQ component is located only in PEO phase. By the following post-treatment, the polymer component was removed and residual cured SSQ structure was formed that could be available as etching mask. In this study, the authors investigated post-treatment process of SSQ in the mixed component system, comparing thermal treatment and oxygen plasma treatment. A significant difference was observed in the vertical shrinkage of the film thickness formed on a Si wafer. The plasma-treated film shrank to 72% of the baked film. By the examination of behavior of PS phase and PEO phase during post-treatment, respectively, it was found that the difference was caused by the curing behavior of SSQ in PEO. In the case of plasma treatment, the polymer component was removed before SSQ curing. As a consequence, plasma treatment gives voidless structure. Using the template as etching mask, 16 nm pitch pattern was transferred to Si wafer.