Pipelines for natural gas transmission may be subjected to plastic deformation by the outside force such as ground subsidence, ground liquefaction, cold bending and mechanical damage. Plastic deformation affects the tensile properties and fracture toughness. Tensile test, Crack tip opening displacement test and Charpy impact test were conducted on an API 5L X65 pipe to investigate the mechanical properties of pipeline subjected to plastic deformation. Axial tensile pre-strain up to approximately 10% was applied to plate-type tensile specimens cut from the API 5L X65 pipe prior to mechanical testing. Tensile test revealed that the yield strength and the tensile strength increased with increasing tensile pre-strain. However, the CTOD and Charpy impact energy values for crack initiation decreased with increasing tensile pre-strain.