Abstract: The aqueous tea solution extracted from green tea was used to dye cotton fibers by post-mordanting method with tannic acid, Zn(CH3COOH)2, and KAl(SO4)2 as mordanting agents. The effect of pH value of dye bath and mordant concentration on the dyeing of fibers were analyzed. The transmittance decrease percentage of liquor was also tested after dipping the dyed fibers with a certain time. TM-1000 was used to analyze the microstructure of fiber samples. In addition, the strength of fibers before and after dyeing were also tested. The results showed that the K/S value was the maximum but the color fastness was inferior when dyed with Zn(CH3COOH)2 as a mordanting agent. The characteristic groove structure was clearly seen in the surface of tannic acid dyed cotton fibers. Insoluble complex was deposited on the surface of Zn2+ and Al3+ dyed fibers. In comparison with the cotton fibers before dyeing, the strength decreased especially dyed in acid condition, and elongation increased for the dyed fibers.