In order to establish characteristic indexes of floc structure in activated sludge, 19microscopic parameters used for description of floc structure were divided into four groups: floc size (SZ), compactness (CP), regulation (RG) and filamentous microbes (FL). These four groups included 4, 5, 8, 2indexes, respectively. Principal component analysis method (PCA, linear dimension reduction) and Isometric mapping method (Isomap, nonlinear dimension reduction) were used to reduce dimensions of these parameters of floc structure. By comparing decrease range and effectiveness of dimension reduction with two methods above, the characteristics indexes of floc structure were established. After treatment of dimension reduction with PCA, the group index of SZ, FL of floc structure can be characterized by 1comprehensive index, so can the group index FL, but for the group index CP, RG, each of them need 3comprehensive indexes to represent their characteristics. The decrease range of dimension reduction of SZ, CP, RG, FL are 0.750, 0.400, 0.625 and 0.500, respectively. The dimensionality of floc structure reduced by Isomap method can be characterized by 1comprehensive index for each group, the decrease range of dimension reduction of SZ, CP, RG, FL are 0.750, 0.800, 0.875 and 0.500, respectively. Therefore, the comprehensive indexes with Isomap dimension reduction are more accurate, concise to describe floc structure characteristics than those with PCA dimension reduction and more suitable for being characteristics indexes of floc structure in activated sludge.