Natural fibers such as silk, cellulose, and flax have recently attracted considerable attention as a low-cost, eco-friendly alternative to glass reinforcements in polymer composites. However, the high level of moisture absorption and poor resin wettability of the natural fibers may lead to void formation in the final part. Since the voids are detrimental to the properties of composites, accurate characterization of fiber and void content is particularly important. The commonly used techniques for calculation of void volume fraction in traditional polymer composites rely on matrix ignition in a furnace (ASTM D2584). Thus, the matrix is removed from composite samples, and the weight fraction of remaining fibers is obtained for the calculation of void content. However, natural fibers undergo a complete decomposition during the burn-off process due to the presence of oxygen, thus making this technique impractical for natural fiber composites. This paper proposes a new technique to accurately quantify both fiber and void volume fractions of natural fiber composites. This technique involves the separate pyrolysis of natural fibers, epoxy resin, and natural fiber/epoxy composite in a pure nitrogen atmosphere. The remained fractional residuals of these materials are used for determination of fiber and matrix weight fractions. Consequently, the void volume fraction is calculated by having weight fractions, as well as the density of natural fiber, epoxy resin, and composite sample. To validate this new method, a number of silk/epoxy composites with various void levels were manufactured. The pyrolysis is performed by heating samples from room temperature to 800°C at a rate of 10°C/min. Then, the fiber and void volume fractions are calculated and compared with those obtained by the commonly used methods. The results, corroborated by SEM images, demonstrate that the proposed method enables an accurate determination of the fiber and void contents in natural fiber composites.