Although the synthetic organic polymers have been widely applied as flocculant aids to improve solid-liquid separation efficiency, it is not very clear how the charge type and molecular weight of the polymers influence the characteristics of flocs. In this paper, floc characteristics, such as floc size, density, structure, strength and reversibility were investigated in details in terms of their response to the polyacrylamide (PAM) with different charge types and molecular weights. Laser light scattering, image analysis and settling technology were employed simultaneously. The results showed that floc size induced by anionic PAM with medium molecular weight (A130) and ultra high molecular weight (A115) was 419 mu m and 438 pm respectively, correspondingly, floc size induced by cationic PAM with medium molecular weight (C448) and very high molecular weight (C498) was 731 pm and 796 mu m. Meanwhile, floc fractal dimension was inversely proportional to polymer molecular weight and the values were between 1.91 and 2.40. In addition, floc density was affected by molecular weight of anionic PAM more dramatically than that of cationic PAM. For a given floc size, the density increased with molecular weight. Furthermore, flocs formed by cationic PAM showed much greater shear-resistant ability and reversibility than anionic PAM. Nevertheless, the poor reversibility of the flocs formed by high molecular weight was also observed. An explanation for the resultant floc characteristics was offered in terms of flocculation mechanisms. (C) 2009 Elsevier B.V. All rights reserved.