Small-angle X-ray scattering has done to explore the difference in the strength of the thermal fluctuation effects between A-B diblock copolymer and (A/2)(2)-B Y-shaped copolymer. SAXS profiles for both systems in their disordered states were analyzed with the Brazovskii-type non-mean field theory by Fredrickson and Helfand. The analyses yielded Ginzburg number characterizing the strength of thermal fluctuation effects. Ginzburg number of Y-shaped copolymer is larger than that of diblock copolymer, suggesting that the strength of the thermal fluctuation effects is affected by the architecture of copolymers, (c) 2008 Elsevier Ltd. All rights reserved.