Two types of polyethylene are used for pipeline systems: medium density polyethylene and high density polyethylene. A polyethylene pipe, being a thermoplastic, softens on heating and hardens on cooling. Because of the viscoelastic nature of polyethylene, stresses decrease under applied constant strain and strains increase under applied constant stress. To account for the complex behavior of polyethylene pipe, laboratory tests were performed to study its thermal and mechanical properties. This paper presents the test results, findings, and design recommendations for polyethylene pipe properties. The tests were performed in a temperature controlled room, where properties were investigated for thermal variations expected in the field. Two types of tests were performed: stress relaxation tests and temperature ramp tests. Test methods and properties are summarized for relaxation modulus, instantaneous modulus, Poisson’s ratio, thermal expansion/contraction coefficient, and stress relaxation rates.