Electron irradiation-induced phase change in tungsten was examined by in situ high resolution electron microscopy. In samples prepared by a chemical polishing technique using a NaOH aqueous solution, an alpha to beta phase change was induced under 200 keV electron irradiation at 773 K. It is suggested that sodium atoms deposited on the tungsten sample from the solution, rather than oxygen atoms, may play an essential catalytic role in inducing the phase change.