Using artificial cores and polymer solutions with such factors as polymer concentration, salt concentration and molecular weight, core floods were performed to examine the influence of viscoelastic properties of polymer solutions on flow behaviour. The experimental results show that when the injection rate excedes a critical injection rate, elastic viscosity becomes dominant. This critical injection rate is dependent on the polymer concentration, salt concentration and molecular weight. There was a good linear relation between elastic viscosity/shear viscosity and shear rate by means of experimental results. According to the relation, the critical injection rate can be determined.