Nanopores with subnanometer precision were fabricated and fine-tuned in freestanding poly(methyl methacrylate) (PMMA) nanofibers by in situ electron beam irradiation. The nanopores were formed and expanded by focused electron beam irradiation, which was attributed to the sputtering and decomposition of PMMA. Meanwhile, the fabricated nanopores could also be shrunk and closed by low electron intensity irradiation, which may have been caused by surface tension induced mass flow due to the glass transition of PMMA. This work provides a new method to fabricate precise nanopores. It also provides important evidence about the theory of direct nanofabrication by electron beam irradiation. (C) 2008 American Vacuum Society.