Thermally conductive carbon fiber/poly(l-lactic acid) (PLLA) composites were made by using fatty acid amides as binders to form a net-like structure of carbon fibers in them, aiming at achieving high thermal diffusivity of the composites with a small amount of the fibers and facilitating their processing without much cost. Infrared thermography analysis of composites with varying the size of carbon fibers and controlling the solubility parameters of fatty acid amides in them revealed that the composite with 10 wt% of 6 mm-long carbon fiber and 5 wt% of N,N'-ethylene bis-olearamide had a thermal diffusivity comparable to that of stainless steel (SUS304). This high thermal diffusivity was due to percolation networks of long carbon fibers bound by the specific low-polarity amide in the composite.