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Extending the fabrication methodology of solid-state nanopores in a wide range of materials is significant in the fields of single molecule detection, nanofluidic devices, and nanofiltration membranes. Here, we demonstrate a new method to directly fabricate size- and density-controllable sub-10 nm nanopores in WO 3 nanosheets using single swift heavy ions (SHIs) without any chemical etching process. By selecting ions of different electronic energy losses (S e ), nanopores with sizes from 1.8 to 7.4 nm can be created in WO 3 nanosheets. The creation efficiency of nanopores achieves ∼100% for S e > 20 keV/nm, and there exists a critical thickness below which nanopores can be created. Combined with molecular dynamics simulations, we propose that the viscosity and surface tension of the transient molten phase caused by SHIs are the key factors for the formation of nanopores. This method paves a way to fabricate solid-state nanopores in materials with a low viscosity and surface tension.