This study was undertaken to prepare and characterize the newly asymmetric polyimide membranes by using the phase inversion technique. They were performed in ultrafiltration. The casting solution was prepared from the conventional method and the newly developed one. The latter was that the solution was directly prepared from a polyimide solution, which was yielded by the one-step polymerization. The membranes were prepared under several conditions to investigate their separation process. All the membranes have exhibited outstanding chemical stability as well as excellent performance because polyimides as membrane material have shown good solubility, excellent chemical stability, and outstanding thermal property.