Ni0.5Zn0.5Fe2O4 ferrite powders were synthesized by a hydrothermal method. The nanoparticles of these materials were mixed with a thermal-plastic polyurethane (TPU) polymer in order to form a suitable composite for electromagnetic wave absorber in a frequency range from 2.0 to 15.0 GHz. The effect of particle size on the reflection loss was investigated by comparing nanosized and microsized powders. The reflection loss as a function of frequency (f), thickness of the absorber(d), the real and imaginary part of permittivity (epsilon' / epsilon") and the real and imaginary part of permeability (mu'/mu") were obtained by calculation using the Bruggeman effective medium theory. The effect of Co2+, Cu2+, and Mg2+ on the reflection loss was also studied.