The purpose of this study is to analyse the effects of knit samples' mechanical properties on changes of skirt length of knit flared skirt, and to compare it to the knitting method of a seamless knitted flare skirt. So, I compared the appearance and 3-dimensional shape of a seamless knitted flare skirt with those of a fully cut knit flared skirt. For this study, 18 kinds of cut and sew or seamless knitted test garment were made. The data analysis methods used in this study were descriptive statistics, one-way ANOVA, correlation analysis, and regression analysis. The results are as follows: The result of the evaluation of knit samples' mechanical properties by the KES-FB system showed that elongation decreased as gauge increased and course direction was greatest among grain directions. The length change after wearing the knitted flare skirts appeared wholly equal to the seamless knitted, compared with the cut and sew. The change was biggest at 180°, 7G of the cut and sew. In grain direction, bias cut showed more than 1.2cm of length change. As the weight and thickness increased, the length changes become larger. The regression analysis indicated that the greatest contributive variables on the length changes were elongation, weight, and thickness.