This paper aims to examine the correlations between floe physical properties, such as size, density, structure (in terms of fractal dimension) and optimum dosage in an alum sludge conditioning with organic polymer and also dewatering using an air pressure plate apparatus. Initially, optimum dosage was evaluated by a modified specific resistance to filtration (SRF). Thereafter, floe size, density and structure were examined in greater detail by means of an image analysis system. The results of this investigation reveal that: (1) polymer conditioning of alum sludge can result in a significant increase of floe size with a plateau to be reached in higher dosages; (2) higher floe effective densities are associated with larger doses; and (3) increased fractal dimensions are associated with increased dose, particularly in small dosages, this reflecting the increased compactness of floe structure. However, no direct correlation was found between floe physical properties and optimum dose both in conditioning and dewatering except for the weak correlation in floe size and optimum dosage.