Effects of molecular weight (MW), charge density (CD) and structure of polyacrylamide (PAM) on conditioning of excess sludge (ES) and ozonized sludge (OS) with/without ferric salt pretreatment (FSP) were investigated by orthogonal experiment. Rheological and morphological analyses were conducted to elucidate conditioning mechanism of ES and OS. OS had smaller particle size, worse filterability, lower flowability and more evident thixotropy than ES, and required FSP before conditioning. Orthogonal experiment showed that the optimal conditioner was PAM with medium MW and cross-linked structure for ES, and high MW and linear structure for OS after FSP, respectively. The specific resistance of filtration and capillary suction time were reduced to (1.2 +/- 0.1) x10(12) m/kg and 25.8 +/- 1.9 s for OS after FSP and optimized PAM conditioning, and particle size increased from 30.3 for OS to 269.7 mu m after conditioning. The limiting viscosity, structural strength and thixotropy of conditioned OS were lower than conditioned ES, but greatly enhanced by FSP. Morphological analyses confirmed that ozonation cracked microbial cells of sludge, and FSP obtained more porous structure and reinforced floc strength of PAM conditioned OS.