A simple theoretical analysis indicates that high permeability is possible in nanowire arrays with the magnetocrystalline anisotropy comparable to the demagnetization energy and its easy axis perpendicular to the nanowire. Both conditions can be satisfied in Co nanowire arrays. With proper fabrication conditions, we have fabricated Co nanowire arrays with crystalline easy axis perpendicular to the nanowire. For Co nanowire arrays with diameters of 50 nm and inter-pore distances of 90 nm, the real part of the permeability is about 3.5 and the loss tangent is about 0.045 below 4.5 GHz. We have also shown that by magnetizing the sample to different remanent states, its ferromagnetic resonance frequency can be tuned between 4.5 and 6.5 GHz.