In　this　work,　a　critical　transition　at　about　500　degrees　C　in　alternate-current　(a.c.)　conductance　and　dielectric　relaxation　of　synthesized　fused　silica　is　first　observed.　The　synthesized　fused　silica　sample　demonstrates　the　features　of　high　hydroxyl　content　and　low　metal　impurities　through　infrared　absorption　and　ultraviolet　transmittance　analyses.　The　dispersion　profiles　of　conductivity　at　different　temperatures　demonstrate　the　feature　of　the　thermally　activated,　long-range　conduction.　Besides,　the　temperature　dependence　of　a.c.　conductivity　and　dielectric　relaxation　shows　different　behaviors　in　high　or　low　temperature　range,　thus　lead　to　a　critical　transition　occurring　around　500　degrees　C.　It　is　believed　that　the　occurrence　of　the　critical　transition　is　ascribed　to　the　heterogeneous　crystallite　domains,　which　go　through　a　transformation　of　stacking　structure　when　the　sample　was　subjected　upon　a　heating.　(C)　2015　Elsevier　B.V.　All　rights　reserved.