This document describes a method for directly measuring the thermal diffusivity of carbon fibers along their axial direction using laser flash analysis. Bundles of carefully aligned dry carbon fibers are used as specimens. Thermal diffusivity values from a variety of carbon fiber types measured with this new preparation methodology show very low standard deviations. This allows direct measurement of fiber tows without assumptions about fiber-resin interactions needed for other common methods. Results demonstrate the viability and advantages of this new preparation and testing approach for accurate on-axis carbon fiber thermal diffusivity characterization.