Two hybrid powertrain configurations, including parallel and series hybrids, were simulated for fuel economy, component energy loss, and emissions control in Class 8 trucks over both city and highway driving conditions. A comprehensive set of component models describing engine fuel consumption, emissions control, battery energy, and accessory power demand interactions was developed and integrated with the simulated hybrid trucks to identify heavy-duty (HD) hybrid technology barriers. The results show that series hybrid is absolutely negative for fuel-economy improvement of long-haul trucks due to an efficiency penalty associated with the dual-step conversions of energy (i.e. mechanical to electric to mechanical). The current parallel hybrid technology combined with 50% auxiliary load reduction could improve fuel economy by 5-7% in long-haul trucks, but a profound improvement of long-haul truck fuel economy requires innovative technologies for reducing aerodynamic drag and rolling resistance. The simulated emissions control indicates that hybrid trucks produce less carbon monoxide (CO) and hydrocarbons (HC) emissions than conventional trucks. The results further indicate that the catalyzed diesel particulate filter (CDPF) played an important role in CO oxidation besides particulate matter (PM) emissions control. Limited ammonia (NH3) emissions could be slipped from the urea selective catalytic reduction (SCR) designed for reducing nitrogen oxides (NOx), but the average NH3 level are below 20 ppm. Meanwhile the estimations show 1.5-1.9% of equivalent fuel-cost penalty due to urea consumption in the simulated cases.