Abstract: Potassium (K) plays pivotal role on cotton production and fiber quality. Our present study focused on the effects of K on the quality of fiber. Two cotton varieties, Lu18(L18) and Zhongmian 41(ZH41), were used and four rates of K fertilizer were applied (K2O 0，60，180，300 kg/ha) under field condition. The results showed, K application can raise the fiber’s length in both parts of the plant, and increase the final micronaire.. Moreover, with the increase of K application, the magnitude of the increase is getting larger, but it had no effect on the time for the fiber to get the longest length. The K application can raise the 0mm and 3.2mm gauge tenacity on both upper and lower positions. However, the behaviors for different positions were not constantly. In the lower part of the plant, gauge tenacity increase first, then decrease with the increase of K application, but in the upper position, it increased constantly. The application of K significantly enhance the fiber maturation and the magnitude at both positions of the plant, The application of K at 300 kg/ha could significantly increased the fiber maturation at both positions of the plant.