The dielectric properties of epoxy/short carbon fiber composites at different concentrations 0, 5, 10 and 15% by weight, different thicknesses 2 and 4 mm, and frequency in the range from 20 Hz to 1 MHz were characterized. Scanning electron microscopy and differential scanning calorimetry were utilized. The alternating current (ac) electrical properties (complex impedance, dielectric constant, dielectric loss, real part of electric modulus, imaginary part of electric modulus, electrical conductivity, and relaxation time) were determined. It was found that the applied frequency, filler concentrations, and composite thickness affected the ac electrical properties of the epoxy/carbon fiber composites. The dielectric behaviors of the interfacial polarization between epoxy matrix and carbon fibers could be described by the Maxwell-Wagner-Sillars relaxation. The analysis of the complex electric modulus in the frequency range from 20 Hz to 1 MHz revealed that the interfacial relaxation followed the Cole-Davidson distribution of relaxation times. The universal power-law of ac conductivity was observed in the epoxy/carbon fiber composites. The calculated power exponent (near unity) is physically acceptable within this applied model.