In order to characterize cotton fiber length distribution, the probability density function with parameters were used to describe the fiber length frequency histograms. In this paper, four cotton fiber samples (bale, carded sliver, combed sliver and finisher sliver) were selected, and the fiber histograms by weight were measured by USTER AFIS Pro. Two- and three-component mixed Weibull distributions were adopted by us to fit these histograms, and the relevant fiber length measures were calculated. The results showed that mixed model could well describe the entire fiber length distribution of different cotton fiber samples, and two-component mixed Weibull distribution, rather than three-component one, fitted these histograms better.