The asymmetric PS(46k)-b-PMMA(21k) film was spin-coated on a neutral brush layer grafted onto a Spin-On-Glass (SOG) layer on the aluminum oxide layer on a Si substrate. An aluminum oxide layer was used as a hard mask. The PS-b-PMMA film was annealed at 190 degree C under vacuum for 3 days, so that the ~20 nm diameter nanopores in the film was successfully prepared as a result of self-assembly. The nanopores in the PS-b-PMMA film were able to be transferred to the Si substrate by CF4 etching through the SOG and aluminum oxide layers, although the etching was not uniform. The non-uniformity of the etching is also discussed.