Abstract: The effects of shading fruit with opaque paper bag at the late stage of fruit enlargement on the contents of chlorophyll, carotenoid and in “Hongshigan” citrus (C.reticulata × C.sinensis) fruit peel and its color were examined. The results showed that after shading,the chlorophyll content in peel decreased quickly, which resulted in its earlier color shifting. In contrast, the contents of total carotenoids and each carotenoid component did not increase,but decreased significantly. At the stage of fruit riping, both chlorophyll in shaded and unshaded fruit disappeared,and the shaded fruit,owing to its lower level of carotenoids, had a lighter color than the unshaded fruit. The sugar content in peel of shaded fruit did not differ obviously from that of unshaded fruit at the earlier stage, but dropped markedly at the late stage of shading. Removing the enclosing paper bag from shaded fruit at the late stage of shading resulted in the increase of sugar, and correspondingly in the increase of carotenoid, especially of β-cryptoxanthin accumulation with consequent darkening of fruit color.These results stressed the effect of light on stimulating carotenoid synthesis,especially the accumulation of β-cryptoxanthin in citrus fruit peel. The light is the enviromental signal essential for carotenoid synthesis in citrus fruit peel during certain stage of fruit development.