We have measured the complex permeability and permittivity spectra of Co2Z/epoxy composites for different volume concentrations Vc varying from 0.2 to 0.6. The effects of Vc on matching and bandwidth properties have been studied. For Vc ges 0.5, there exist two matching points. The first matching, occurring away from the magnetic resonance region, has its origin in the quarter-wavelength matching, while the second matching is related to magnetic resonance. Both matchings are perfect, i.e., reflectivity |R| (dB) = -infin can be theoretically achieved. For Vc les 0.4, only one matching, the first matching, is found. At the matching point, |R| (dB) is finite. |R| (dB) decreases with decreasing Vc, due to the decrease in tan deltam and epsi'. In addition, with an increase in Vc, the percentage bandwidth DeltaWp increases and the corresponding optimum thickness to decreases. A Co2Z/epoxy composite with Vc = 0.6 can achieve DeltaWp = 90% for reflectivity |R| (dB)< -10 dB at a thickness of 0.30 cm.