Abstract <p>This article deals with the instrumented indentation method as an effective tool for studying the mechanical properties of polyethylenes. This method allows one to determine a number of properties of materials, such as their hardness, modulus of elasticity, and rheological characteristics. We present examples of how the instrumented indentation method can be used to study properties of various types of polymeric materials and describe parameters that can influence measurement results. The capabilities of the method have been studied experimentally, using high-density polyethylene for the manufacture of gas pipelines as an example. The results confirm that the method can be used to study properties of polyethylene (PE) gas pipes for evaluating their engineering performance. We have studied pipe samples after operation for a long time and a new pipe sample. The polyethylene samples differing in service life have been shown to be rather similar in hardness as an indicator of plastic deformation resistance under static load, whereas their rheological properties differ significantly. This feature should be taken into account in performing various mechanical tests, because the loading rate in such tests can have a significant effect on their results. The modulus of elasticity and hardness have been shown to decrease with increasing dwell time under load because of the stress relaxation in the deformed region under the indenter. We have examined how the structure of polyethylene influences its mechanical properties. The modulus of elasticity of the polyethylene samples has been shown to increase by several times as the degree of crystallinity of the material increases from 48 to 56%. The results of this study demonstrate that the instrumented indentation method has considerable potential as an effective means of analyzing the elastoplastic and rheological properties of polyethylenes in relation to their crystal structure.</p>