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An ac calorimetric method is applied to the direct measurement of thermal diffusivity of single fibers. Theoretical analysis is presented, taking into account the two‐dimensional ac thermal wave for cylindrical systems and the effect of time delay at the temperature detecting system. Experimental tests are performed using nickel and stainless steel wires about 20 μm in diameter. The thermal conductivity of fibers about 10 μm in diameter of the advanced materials, polyacrylonitrile‐based carbon, TORAYCA, and aramide, Kevlar, is determined. The relationship between the thermal conductivity and electric conductivity and the crystalline size of these fibers is discussed. For composite materials made of carbon fibers, the observed thermal conductivity agrees with the thermal conductivity estimated using the values determined by the present method. © 1996 American Institute of Physics.