We report magnetoresistance in Cr‐Fe bulk alloys and its variations associated with the microstructure change due to the phase decomposition. Large negative magnetoresistance (MR) is observed in the as‐quenched Cr‐Fe bulk alloy at liquid helium temperature, but not at room temperature. This is attributed to the spin cluster glass in the homogeneous solid solution. A unique feature of the MR in these homogenous alloys is that it does not saturate even in the presence of very high magnetic fields. However, such MR behavior changes when ferromagnetic particles precipitate by annealing. A lower degree of compositional fluctuation of Fe progresses by annealing the alloy at 773 K for 400 h and then a higher degree of decomposition occurs after a prolonged aging for 2600 h. In the annealed specimens, MR is observed even at room temperature. The MR change becomes more prominent at a lower field region, since ferromagnetic particles precipitate from the solid solution. The degree of the phase decomposition of the Cr‐Fe solid solution is quantitatively evaluated by the atom probe analysis, and the origin of the MR dependence on the annealing conditions is discussed. ©1995 American Institute of Physics.