Carbon fiber with diameter in the range of nano to micro meter was prepared by carbonization of polyacrylonitrile (PAN) and polymethylmethacrylate (PMMA) blend fiber which was produced via wet spinning of PAN/PMMA blend solution. At the same technical condition, the high molecular of PAN favored the production of thin diameter of carbon fiber, and the high drawing ratio led to small diameter of the obtained nano/micro carbon fiber. The formation of graphite structure during carbonization was characterized by Raman and X-ray diffraction. The results improved that high temperature of carbonization developed high degree of graphitization and high conductivity of the nano/micro carbon fiber. The composites containing 2-8wt% of the obtained nano/micro carbon fiber were fabricated. The complex permittivity ε’ and ε’ for the composites in the frequency range of 8-12 GHz was measured, and the reflection loss for 180×180×3mm3 composites was also investigated. The microwave absorption property of these composites was improved greatly with increasing content of the nano/micro carbon fibers. At the addition of 8 wt% of the nano/micro carbon fiber, the reflection loss of the composite displayed -16dB at 11.3GHz, and the band with smaller than -5dB was 3GHz. Moreover, the calculated results based on the complex permittivity seem identical with the experimental values.