Polymer nanofibers with controlled diameters were formed by continuous spinning using ordered anodic porous alumina as a spinneret. The diameter of the polymer nanofibers could be controlled by changing the hole size in the porous alumina membrane. In addition, the diameter of the obtained polymer nanofibers could be reduced by mechanical stretching in water at 80 degrees C. The obtained polymer nanofibers are expected to be used for various applications that require nanofibers with uniform diameters.