A wide band microwave characterization of nanocomposites based on commercial multiwall carbon nanotubes (MWCNTs) and epoxy resin is presented. Field Emission Scanning Electron Microscopy is used for morphological samples analysis of nanocomposites and MWCNTs. Complex permittivity is measured using a commercial dielectric probe and a Network Analyzer. A statistical analysis based on one-way analysis of variance (ANOVA) technique is performed. The aim of this statistical analysis is to investigate concentration of nanoparticles inside the polymer matrix. It can be significantly different in nanocomposites even if the samples have similar electrical properties. The reflection coefficient of a single-layer structure backed by a metallic plate was simulated taking into account the value of the measured permittivity and the results compared with the measurements. A two-layer absorber was designed and optimized by means of an annealing algorithm.