The Cu-12.8wt%Fe alloys are prepared in a vacuum induction furnace and then drawn to Cu-Fe composite wires with the drawing ratio of 8.2. The thermal stability of Fe filaments in the deformed Cu-12.8wt% Fe composite wires under different annealed temperature is investigated. The results show that the instability of the Fe filaments in the Cu-Fe composites is controlled by the longitudinal boundary splitting, then the splitting Fe filaments subsequently evolve into the cylinders. The thermal instability of the cylindrical Fe filaments is controlled by the two instability modes of Rayleigh perturbation and two dimensional Ostwald coarsening. The model calculations of two modes indicate that the perturbation breakup of cylindrical Fe filaments firstly occurs at the ones with smaller diameter. The breakup time of cylindrical Fe filaments decreases with the increasing of the annealing temperature. The coarsening diameters of cylindrical Fe filaments increase in linear proportion with the holding time. The smaller is the diameter of cylindrical Fe filaments, the larger is the coarsening rate.