We determined stable carbon, nitrogen, and oxygen isotopic compositions (delta C-13, delta N-15, and delta O-18) of fibers to discriminate between plant, animal, and synthetic fibers. The delta C-13 and delta N-15 values of synthetic fibers are significantly lower than animal fibers. Cashmere and alpaca have relatively lower delta O-18 values than other animal fibers. Moreover, there are significant differences between acrylic and cashmere (e.g., in delta N-15 values). These results suggest that the stable carbon, nitrogen, and oxygen isotope analysis would be useful in discrimination of natural and synthetic fibers.