We demonstrated a simple and time-efficient processing method for facilitating a microphase separation of block copolymers (BCPs) based on a single step of spin casting with low volatile solvent and in situ annealing.Well-ordered lamellar patterns of poly(styrene-b-methyl methacrylate) BCP films having wide range of molecular weights (51 ～ 235 kg/mol) were fabricated by a single 3-minute process of spin-casting even without the conventional pretreatment of substrate neutralization.The formation of this well-ordered lamellar structure is attributed to a synergetic effect between slow solvent evaporation and thermal energy that may provide an efficient cooling profile for the BCP film during the spin-casting process.