Multi-walled carbon nanotubes (MWNT) are characterized at X-band (8-12 GHz) by waveguide measurements. The scattering parameters (S-parameters) of thin MWNT papers containing a large ensemble of randomly oriented nanotubes are measured by a vector network analyzer. An extraction algorithm has been developed to compute the effective complex permittivity (ε= ε-j ε'') and permeability (μ = μ' -j μ'') of the MWNT papers from the measured S-parameters. The extracted effective medium parameters are verified by finite-element simulations using Ansoft HFSS. The uncertainties for this characterization method are analyzed. The extracted conductivity of the MWNT papers is on the order of 1500 S/m and the dielectric constant is on the order of-500. No significant magnetic responses are observed. To overcome the relatively large uncertainty associated with the reflection coefficient S11, the extracted imaginary permittivity (ε'') is fitted by the Drude model and the dielectric constant (ε') and permeability are recalculated.