Main topics are the development and the build-up of an 18ton hybrid truck with a parallel hybrid drivetrain. With this truck it is possible to drive up to 3 kilometers in the pure electric driving mode. In this R&D Project the Engineering Center Steyr, a company of MAGNA Powertain has developed a hybrid truck with an integrated motor generator (IMG) including a clutch system for pure electric driving. Also a Li-Ion power battery, electrified traction voltage auxiliaries (steering pump, air compressor and climate compressor) and a cooling system for the new power electric components.The beginning of the paper shows a detailed layout of the developed hybrid truck. Chapter two shows the developed simulation model which is used for the dimensioning and for detailed simulations of hybrid drivetrains. To simulate thermal effects, a co-simulation environment between a longitudinal dynamics and thermal management software is applied. The longitudinal dynamics software calculates the propulsion power which determines the thermal load of the components. Particular attention was given to the electrical and thermal behavior of the traction battery. The coupling of longitudinal and thermal simulation enables a realistic computation of the energy use, since the effects such as the change of efficiency by temperature and load can be taken into account. This behavior has a high influence in the development and tuning of the vehicle's components and operating strategy. Therefore a special focus is given to the operation strategy of hybrid commercial vehicles. Finally an application shows simulated data of the hybrid commercial vehicle on standardized driving cycles and also cycles with a high proportion of urban driving. These simulations show a meaningful influence on the temperature of the components, especially the battery.To gain high market shares for hybrid trucks the total cost of ownership will be the main driver, so the development of affordable hybrid components (battery, traction system and auxiliaries) with high efficiency and the development of optimized overall hybrid vehicles systems have to be the short term objective.