Nanoimprint lithography is a promising technology for patterning large-area structures in nanometer scale at a low cost. In order to fabricate large-area nanoimprint master mold, interference lithography is widely used in defining periodical structures. However, neither roughness nor structural dimension can be effectively controlled via interference exposure. In this paper, we report a fabrication technique based on V-shaped master mold that can adjust line width of gratings as well as reduce the sidewall roughness. The fabrication of the V-shaped grating master mold is demonstrated, and the line width tuning and smoothening processes are discussed. With the help of the smoothening process, the optical efficiency of smoothened guided-mode resonance grating increased by 75 % from the original sample.