Thin-film β tungsten (β-W), a metastable phase of tungsten, holds significant potential in the fabrication of superconducting and spin-memory devices. However, due to the rapid surface passivation of tungsten in oxygen and moisture, the synthesis of nanosized metastable β-W with the intrinsic atomic surface is still difficult, and their magnetic properties remain rather unexplored. Inspired by the strong host-guest interaction-induced stabilization, we reported the synthesis of atomically thin (1.0-1.3 nm) metastable β-W nanowires within single-walled carbon nanotubes (SWCNTs) through an oxygen-assisted transformation of starting W