The paper deals with the effect of the velocity gradient (in range of 28.4 ­ 307.2 s-1) on properties (size and structure) of aggregates formed during drinking water treatment. Further, it describes the influence of these properties on the separation by sand filtration. The formation and filtration of suspension proceeded in a pilot plant (mixing tank, rapid gravity filter) and iron sulphate was used as a coagulant. It was proved that with increasing velocity gradient, the aggregate size decreases, aggregate size distribution narrows (suspension is more homogeneous), fractal dimension D2 increases (aggregates are more compact) and fractal dimension Dpf decreases (aggregates are more regular). Contrary to large and porous aggregates (d ~ 1 mm and D2 = 1.58) formed at G around 30 s-1 (filtration run length around 30 hours), the smallest and most compact aggregates (d ~ 50 µm and D2 = 1.9) formed at G > 200 s-1 proved the best filterability (filtration run length up to 78 hours).