Polymethyl methacrylate (PMMA) microstructures were fabricated by a polymeric microreplication technology using ultrasonic vibration energy. A commercial ultrasonic welder system was used to apply ultrasonic vibration energy for micromolding. Two different types of nickel micromolds, which were equipped with pillar-type and pore-type microstructures, were fabricated. PMMA was used as the polymer microreplication material, and the optimal molding times were determined to be and for the pillar-type and pore-type micromolds, respectively. Compared with conventional polymer microreplication technologies, the proposed ultrasonic microreplication technology showed an extremely short processing time. Heat energy generated by ultrasonic vibration locally affected the vicinity of the contact area between the micromold and the polymer substrate. Consequently, only that very limited area was melted so that the bulk material was not seriously affected by the thermal effect and thermal shrinkage could be minimized. Furthermore, although the replication process was not performed in vacuum conditions, the ultrasonic micromolding showed high fidelity in polymer microreplication using the pore-type micromold.