Mossbauer and magnetic measurements were made on Fe-Co particles precipitated in a Cu matrix in order to investigate magnetic properties of fcc Fe-Co alloys around the concentration for the ferromagnetic to antiferromagnetic transition, using the samples Cu 98 (Fe 1- x Co x ) 2 with x =0.3 ∼0.45. The antiferromagnetic fcc Fe-Co with a small moment was observed at x =0.3, whereas the ferromagnetic fcc Fe-Co with nearly the full moment appears and coexists with the antiferromagnetic fcc Fe-Co for x =0.35 ∼0.4. The antiferromagnetic Fe-Co disappears around x =0.45. This sharp magnetic transition associated with a small change in the composition is quite similar to the case of Fe-Ni Invar alloys.