The properties of fiber reinforced composites are mainly determined by the fraction of reinforcement and matrix. Thus, to design a system based on composite materials, it is vital to carefully measure the volume fractions of the composites with a proper method. Digestion by strong acid or ignition at high temperature in an oxidizing environment are conventional for measuring content fractions. In essence, these methods assume that the reinforcement does not lose weight by digestion or ignition. However, by neglecting reported vulnerable oxidization characteristics of carbon fiber, these conventional methods result in inaccurate fiber volume fractions of carbon fiber/epoxy composites. In this study, an effective and accurate method, having only 2 steps in measuring process and 1.5%p maximum error, for determining the fiber volume fractions of two different PAN (Polyacylonitrile)-based carbon fiber reinforced composites via thermo-gravimetric analysis was developed and subsequently verified using the results from 80 microscopic images.