To provide a fiber having thermal emittance large at high temperature and small at low temperature; and to provide a fiber structure and a fiber product.

The fiber having the thermal emittance changed by the temperature contains particles of a perovskite oxide or the like, containing Mn and represented by A1-XBXMnO3 (A is at least one of rare earth metal ions of La, Pr, Nd and Sm; B is at least one of alkaline earth metal ions of Ca, Sr and Ba) and having the thermal emittance large at the high temperature and small at the low temperature.