The purpose of this study was to propose low gas pressure effects on lifetime of natural gas high-density polyethylene (HDPE) pipes by thermal-oxidative aging. The new method to assess the lifetime of HDPE natural gas pipes is based on gas pressure testing. An approach to monitor oxidative induction time (OIT) has been used to predict lifetime. Natural gas HDPE pipes were used to evaluate the effects of low gas pressures on oxidative induction time. In order to emphasize the pressure effects, relatively low temperatures at 45, 55, 65, and 75 °C were utilized for the exposure. The low-pressure conditions were created using air at levels of 0, 0.1, 0.2, 0.3, and 0.4 MPa. The property of high density polyethylene pipes was effectively monitored using the low pressure OIT test. The results show that the aging reaction rate of high density polyethylene pipes increased exponentially with temperature and gas pressure according to the Arrhenius equation. Analytical models were developed to predict the aging reaction rate and lifetime of natural gas HDPE pipes.