Produced water from polymer flooding was much more difficult to be treated than that from water flooding in many oilfields. The simulated experimental results showed that the small initial sizes of oil droplets were the main reason that caused the difficulty in treating the produced water from the polymer flooding process. Polymer hydrolyzed polyacrylamide (HPAM) increased oil-water separation when its concentration was less than 790 mg l(-1). The flocculating effect by the polymer was found to play a dominant role at low HPAM concentration, even though it increased the wastewater viscosity and water film strength. Enhancing the coalescence of oil droplets and shortening rising time of oil droplets were the critical factors affecting the treatment of produced water. A novel crossflow oil-water separator with coalescence and separation sections was exploited. The coalescence section accelerated the coalescence of the small oil droplets and the separation section shortened the rising time of oil droplets. The in situ experiments showed that the separator successfully treated the produced water from the polymer flooding. When the polymer concentration was below 410 mg l(-1), the quality of the treated water after the separator can meet the water requirement for the subsequent filtration processes. (C) 2002 Elsevier Science B.V. All rights reserved.