Polymer flooding is one of the methods to improve sweep efficiency and reduce water mobility when water channeling takes place in an oil reservoir. Theoretically, if the polymer viscosity increases, the mobility ratio decreases. Thus, the oil sweep becomes more efficient while the Recovery Factor (RF) becomes higher. However, there is a phenomenon in which polymer with higher viscosity does not always improve oil recovery. One of the factors that influence this phenomenon is the existence of Inaccessible Pore Volume (IPV), so this study is needed to determine the relationship between polymer rheology and the amount of IPV. Two commercial polymers with the same concentration, FP3630S and ChemEOR, were done by rheology testing and injected into several sandstone Berea cores. The effluents of salt tracer (potassium chloride) and polymer flood were collected, and their concentrations were measured using atomic absorption spectroscopy (AAS) and UV-Vis spectrometry, respectively. The determination of IPV is based on the trailing edge method. Based on Rheology test in the same concentration, polymer ChemEOR has a higher viscosity, but from the Coreflood test, ChemEOR has smaller oil recovery than FP3630S. The IPV of ChemEOR and FP3630S were 34 % and 28%, respectively. The size of IPV of a polymer is influenced by the ability of the polymer to increase viscosity, so that the greater the value of the viscosity given, the greater the value of IPV from the polymer. The FP3630S polymer can reach larger rock pores even though in terms of the water-oil mobility ratio is smaller than ChemEOR. With a smaller IPV, the result proves that FP3630 polymer displays an increase of oil recovery compared to ChemEOR polymer.