An electric field was applied to reactive bilayer thin films consisting of poly(methyl methacrylate-ran-glycidyl methacrylate) (PMMA-GMA) and polystyrene (PS) with various amounts of monocarboxylic acid end-functionalized PS (PS-mCOOH). The fast growing wavelength (A..) of the interfacial fluctuation under an electric field decreased sharply and approached a steady value with increasing amounts of PS-mCOOH. This is because of the decrease in interfacial tension (gamma) between PMMA-GMA and PS resulting from the formation of PMMA-g-PS copolymers in-situ from the reaction between two functional groups of GMA and mCOOH. The reduction of gamma with increasing amount of PS-mCOOH was verified from a reduction in the dispersed domain size. For a reactive bilayer where Y-shaped graft copolymers were formed, the pillars with flattened top surface were observed even when they did not touch the upper electrode. This result is in contrast to that seen for nonreactive bilayer or even other reactive bilayer where diblock copolymers were generated. Also, the maximum pillar height obtained for the reactive bilayer generating graft copolymers was smaller than that for nonreactive bilayer as well as other reactive bilayer generating diblock copolymers. The mechanism of the growth of the pillars of the reactive bilayer was discussed in terms of the chain architecture of copolymers formed in-situ at the interface.