Abstract: An analysis is made about Descartes rays of transparent round-section fibers——a special internal surface reflection rays of the fiber when parallel rays strike the fibers perpendicularly to fiber axis.Production principle,character and primary influence factors of Descartes ray are studied by experiments on the basis of theoretical analysis.It is proved that round-section fiber has Descartes rays,and its principle and characteristics are similar to the water drops producing the rainbow in the sun.