The effects of the host crystal structure of an SrTa4O11: Er3+/Yb3+ up-conversion (UC) phosphor on its luminescence properties were investigated using the tetragonal tungsten bronze (TTB) and hexagonal polymorphs of the SrTa4O11 host. Hexagonal SrTa4O11:Er3+/Yb3+ showed bright luminescence, whereas the UC emission intensity of TTB SrTa4O11:Er3+/Yb3+ was very weak, corresponding to approximate to 1% that of the hexagonal phase. This result was attributed to the local crystal structure around the rare-earth ions in the host lattice.