Polymer flooding is the most successful chemical method to enhance oil recovery. This technique has achieved large-scale field application in China. Polymer flooding is also being tested in several heavy oil fields. For oil field applications, partially hydrolyzed polyacrylamide (HPAM) is the most widely used polymer. HPAM must maintain high viscosity to achieve good oil recovery. In reality, however, polymers can be severely degraded by shearing and heat in the underground environment. This phenomenon is studied in this paper. The two HPAM test samples have molecular weight of 10 million and 20 million. The samples were added to water to achieve concentrations of 500 to 3,000 ppm. The first group of tests was conducted to study the effect of shearing on HPAM viscosity. The second group of tests was carried out to evaluate the effect of high temperature up to 90 °C on HPAM viscosity. It was discovered that both shearing and high temperature can severely reduce polymer viscosity. Data analysis shows that the empirical power law parameters are strong functions of polymer concentration and temperature. Simple formulas are proposed to correlate HPAM viscosity with shearing rate, polymer concentration and temperature. These formulas are useful tools for predicting HPAM viscosity in field applications.