Despite the importance of the effect of pressure on the flow properties of a polymeric material, it is often overlooked also because of the difficulties involved in the experimental measurements. In this study, the effect of pressure on viscosity for an isotactic polypropylene was characterized in both a direct and an indirect method. In particular, a homemade device was adopted to obtain data of viscosity under high pressure and high shear rates. In addition, an indirect method based on the Simha–Somcynsky equation of state was adopted to obtain the dependence of free volume on temperature and pressure on the basis of experimental specific volume measurements; the Doolittle equation was then applied to verify the dependence of viscosity on free volume. The two methods provided similar results, confirming that, at least for polypropylene, the indirect method based on specific volume measurements can be used instead of the more complex direct measurement of the viscosity under pressure.