Polystyrene-b-poly(L-lactide) (PS-PLLA) diblock copolymers with high and low molecular weights were synthesized to form strongly and weakly segregated thin-film morphologies, respectively (i.e., ordered lamellar and disordered textures). Large-sized, well-oriented lamellar microstructures of PS-PLLA thin films were produced for both samples by using the crystallizable solvents: benzoic acid (BA) and hexamethylbenzene (HMB). In strongly segregated (high Mn) PS-PLLA, the oriented microstructures were obtained through the PS-PLLA microphase separation due to directional crystallization of crystallizable solvents at the eutectic point (directional eutectic solidification) regardless of the PLLA crystallization. Lattice matching between crystalline substrates and PLLA was found to be nonessential for inducing microstructure orientation although it may improve its orientation order. In weakly segregated (low Mn) PS-PLLA, the oriented microstructures were formed through the directional crystallization of PLLA on the substrates (crystallization-induced orientation) regardless of the directional eutectic solidification and lattice matching. Two mechanisms: of directional eutectic solidification and of crystallization-induced oriented microstructure have thus been identified. Oriented, defined lamellar trenches can be prepared by hydrolysis of PLLA component, providing a possible path to prepare nanopatterned templates with lamellar nanochannels.