Photo-deterioration of natural fibers (silk and cotton) at the early stage was surveyed with regard to the changes of physical properties and chemical structure of the fibers. Photo-deterioration at the initial stage brought about yellowing and lowering of elongation and strength for both fibers. The changes in elongation and strength of silk induced by UV light, xenon lamp, and sun light showed that the steep decrease occurred at the early stage, followed by the gradual decrease in both strength and elongation, the latter being more remarkable than the former. However, for cotton, the decrease in strength was more pronounced than that in elongation and both changes were smaller as compared with silk. Furthermore the changes in elongation and strength of cotton were simply proportional to exposure time. Regarding the changes in chemical structure, the formation of carbonyl groups by oxidation of cotton fibers was confirmed by copper number measurement. With silk the decrease of tyrosine and tryptophane was found and related to yellowing phenomena.