Fe-Pt-B nanocomposite magnets have attracted much attention because of their excellent hard magnetic properties, in which the face-centered-tetragonal FePt (L1₀) phase ensures high coercivity (_iH_c) and the Fe₂B phase provides high magnetic saturation. A high _iH_c, however, is hard to reach at low Pt concentrations in these nanocomposite magnets. It is known that a high concentration of B favors the formation of L1₀ phase in Fe-Pt-B alloys with low Pt concentration, but the annealed microstructure is usually coarse-grained due to their low amorphous-forming abilities, and the magnetic properties get deteriorated. Replacement of Fe with Co is expected to enhance the amorphous-forming ability of Fe-Pt-B alloys with low Pt and high B concentrations, and to improve their magnetic properties. In this work, the structure and magnetic properties of as-quenched and annealed Fe_55-xCo_xPt₁₅B₃₀ (x=0~45, atomic fraction, %) alloys have been investigated. Melt-spun ribbons were prepared by melt spinning, followed by vacuum annealing at different temperatures. The structure and magnetic properties of the samples were examined by XRD, TEM and a vibrating sample magnetometer (VSM). The results indicate that single amorphous phase is formed in the alloys at x=15~45. After appropriate annealing, a nanocomposite structure consisting of L1₀ and (Fe, Co)₂B phases is obtained at x=0 and 15, and an additional (Fe, Co)B phase gets formed at x=30 and 45. A fine microstructure with mean grain size of ~18 nm has been obtained in the annealed alloys with x=15~45. In these nanocomposite alloys, the best hard magnetic property with an energy product of 94.4 kJ/m³ is reached at x=15. With increasing Co content, the _iH_c gradually increases to a maximum value of 413.7 kA/m at x=30, and then decreases at higher Co contents, which are attributed to the change of the magnetocrystalline anisotropy in L1₀ phases with different c/a ratios.