This paper investigates the aging performance of the lithium ion cobalt oxide battery pack of a single shaft parallel hybrid electric vehicle (HEV) under different ambient temperatures. Varying ambient temperature of HEVs results in different battery temperature and then leads to different aging performance of the battery pack. Battery aging is reflected in the increasing of battery internal resistance and the decreasing of battery capacity. In this paper, a single shaft parallel hybrid electric vehicle model is built by integrating Automotive Simulation Model (ASM) from dSPACE and AutoLion-ST battery model from ECPower to realize the co-simulation of HEV powertrain in the common MATLAB/Simulink platform. The battery model is a physics-based and thermally-coupled battery (TCB) model, which enables the investigation of battery capacity degradation and aging. Standard driving cycle with differing ambient temperatures is tested using developed HEV model. The variation of capacity fading rate under different temperatures is observed. Under extreme temperature conditions, it is found that the range of the voltage response of the battery pack is wider under lower temperatures indicating an increase in internal resistance. The aging of the battery, observed from the decreasing of battery capacity, is also faster under lower temperature. The simulation results of this study provide useful information and reliable model for the effective design of HEV battery management systems.