Abstract: The relations between stable pressure drop and flow velocity of high molecular weight HPAM solutions were measured over a wide range of flow velocity through core simulation tests at 70℃. It was revealed that HPAM solution possesses dual rheological behaviors in porous medium, i. e. a shear thinning pseudoplasticity at lower flow velocity and a shear thickening viscoelasticity at higher flow velocity.Darcy's equation was extended to power law fluids by incorporating the rheology measured by a rheometer and the bulk rheological model into the bundle of capillary tubes pack which decsribes porous medium.Hirasaki's apparent shear rate equation was modified and the relational expression between shear rate in porous medium and Darcy's now velocity was obtained, and finally a in -situ rheological model for HPAM polymer solutions was constructed.