Abstract: In order to study the compatibility of polymer molecular size with porous throat in reservoir,hydrodynamic radius(Rh) of several kinds of polyacrylamide(HPAM) used in polymer flooding in Nanyang Oilfield was measured with dynamic light scattering(DLS).According to the "bridging" principle,the relationship of hydrodynamic radius(Rh) of the polymer molecule can form stable block to the pore throat and radius of the pore throat(R) would be:Rh≥0.46R.Generally,median of average radius of the pore throat(R) in most zones varied from 4 μm to 16 μm in polymer flooding,Henan Oilfield.There were three types of Rh distribution for HPAM products used in oil displacement.Firstly,the Rh distribution of HPAM was quite centralized and it was difficult for this kind of HPAM with few big size polyacrylamide molecules to block formation.Secondly,the Rh was very wide and big size molecules and a lot of impurity (microgel) could easily block formation.Thirdly,although molecular weight of HPAM was low,the Rh distribution was still wide with two distribution peaks located at two ends.As a result,the thickening ability of those polymer molecules with ultra-low molecular weight was too low,and the others with high molecular weight could block the formation.Therefore,formation block would appear if the quality of polymer product was low,or polymer solution was prepared with fresh water when polymer flooding was applied in the reservoirs with low permeability.