The phase transitions for the films of a symmetric polystyiene-b-poly(methyl rriethacryltate) (PS-b-PMMA) on it modified surface, like the order-to-disordcr transition (ODT) in the weak segregation regime, was investigated by in situ grazing incidence small-angle X-tay scattering (GISAXS). The selective interactions at the surface by PS-brush substrate that favors the preferential interactions with the PS component of the block copolymer enhance the parallel orientation of the larnellar microdomains to the Film surface This effect for weakly interacting PS-b-PMMA Films leads to a gentle decrease of the transition temperature with increasing film thickness up to 50L(0)(lamellar period) We discuss this thickness dependence of transition temperatures for PS-b-PMMA films on preferential surfaces in terms of the temperature dependence of chi between two block components