20 nm diameter Fe-Co nanowire arrays with different composition were fabricated by electrodeposition. A nanoporous anodized aluminium oxide film was used as the substrate. The microstructure and magnetic properties were studied by x-ray fluorescence, transmission electron microscopy, x-ray diffraction and vibrating samplemagnetometry. Both the coercivity Hc and the spontaneous magnetization Ms increase when the Co content in Fe-Co alloy increases from 0 to about 30 at.%, and then decrease with further increase in the Co content. A model called the 'chain of crystals' is developed and solved to explain the experimental results.