The flocculation performances of different molecular weight of polydiallyldimethylammonium chloride (polyDADMAC) in the treatment of pulp and paper mill wastewater were studied. The molecular weights used were 8.5 x 10(4), 8.8 x 10(4), 10.5 x 10(4) and 15.7 x 10(4) g/mol. The flocculation performance test was carried out in jar tests with polyDADMAC dosages ranging from 0.4 to 2.0 mg L-1. The test was conducted with rapid mixing at 200 rpm for 2 min, followed by slow mixing at 40 rpm for 10 min and a settling time of 5 min. The effectiveness of the flocculation was measured based on the reduction of the turbidity, the total suspended solids (TSSs), chemical on demand (COD) reduction and zeta potential measurements. It was found that the flocculation performance of higher molecular weight samples was more efficient compared with that of lower molecular weights. This might be due to the bridging mechanism, which occurred concurrently with the charge neutralization effect during flocculation. (C) 2010 Elsevier B.V. All rights reserved.