Abstract: The free-space method is a suitable analysis method for measuring electromagnetic parameters of composite materials, due to its easy changeable incident angle and polarization of electromagnetic waves. This method was used to measure complex electromagnetic parameters of polymers and composites. In order to solve the multi-valued problem caused by the calculation process, several effective ways were proposed for the theoretical analysis. The complex permittivity and permeability of four samples of polymer composite materials were measured in the X-band(8.2~12.4GHz) by using the free-space method. The experimental data are validated with the help of the waveguide technique. The results indicate that the complex electromagnetic parameters of composite materials can be measured exactly by the free-space method.