Cross-linked polymer flooding can increase oil recovery 10 to 20% over conventional waterflooding, while using a lower concentration of polymer than conventional polymer flooding. Based oil production data from a developed oil field, the average incremental oil recovery by polymer flooding is only 10%, indicating that much of the oil is bypassed. We need to know how this bypassed oil is distributed to design an optimal development strategy. Numerical simulations have been performed to study the mechanisms of polymer flooding. For the coarsening downward sedimentary cycle, the simulation model was divided into five layers with average geometric permeabilities of 100 mD, 300 mD, 500 mD, 980 mD and 2,190 mD, respectively. Five spot well patterns with 180 m distance between each injector and producer has been used. The injecting well was controlled by an injection rate of 100 m(3)/d and shut down as the water ratio reached 98%. Crosslinked polymer injection was introduced after the water cut was as high as 90% and stopped as the water cut reached 98%. It was found that most of-the residual oil after polymer flooding existed in small areas far away from the diagonal line for heterogeneous reservoirs, and in layers with low or intermediate permeabilities for cycle reservoirs.