While significant progress has been made in recent years to develop hybrid and battery electric vehicles for passenger car and light-duty applications to meet future fuel economy targets, the application of hybrid powertrains to heavy-duty truck applications has been very limited. The relatively lower energy and power density of batteries in comparison to diesel fuel and the operating profiles of most heavy-duty trucks, combine to make the application of hybrid powertrain for these applications more challenging. The high torque and power requirements of heavy-duty trucks over a long operating range, the majority of which is at constant cruise point, along with a high payback period, complexity, cost, weight and range anxiety, make the hybrid and battery electric solution less attractive than a conventional powertrain. However, certain heavy-duty applications, such as Class 6-7 urban vocational trucks, can benefit from hybridization due to their transient operating profiles and relatively lower vehicle weight. While many studies have quantified the fuel consumption benefits of hybridization in this segment, very few studies have outlined the arduous process of selection and sizing of hybrid powertrain components based on the trade-offs between fuel consumption, payback period, cost, weight, packaging, emissions and aftertreatment temperature. To investigate the potential for electrification in heavy-duty applications, FEV has developed a system level approach for the selection and sizing of heavy-duty diesel hybrid powertrain components using GT-SUITE. The approach has been applied for a Class 6-7 urban vocational truck, which typically experiences low speed driving with frequent start-stops. A dynamic model for the baseline vehicle was developed and calibrated to test data that included, fuel efficiency, engine-out NOx, engine-out PM and aftertreatment system temperature. The model was then updated with hybrid powertrain components and evaluated over cycles developed for chassis dynamometer testing of heavy-duty vehicles, specifically the Heavy Heavy-Duty Diesel Truck (HHDDT) schedule and EPA Urban Dynamometer Driving Schedule (HDUDDS). In the evaluation, key trade-offs were identified between fuel consumption, initial cost, payback period, package size, emissions and vehicle weight. The trade-off analysis demonstrated that similar fuel consumption benefits with an identical payback period could be achieved with multiple hybrid powertrain configurations, however package size, initial cost and weight considerations determined the final optimum solution. The final hybrid powertrain configuration for a Class 6-7 urban vocational truck proposed from this study demonstrates a 20.7% fuel consumption reduction when comparing to the baseline vehicle and applying a two year payback period. In addition, the diesel hybrid powertrain configuration provides an 11% reduction in engine-out NOx emissions and an 86% reduction in engine-out PM emissions, while maintaining aftertreatment temperature of the baseline configuration.