Minimizing working time of the internal combustion engine is an important method for hybrid electric vehicles to save energy and reduce emissions. However, vibrations caused by frequent engine starts and stops deteriorate driveability and ride comfort. Especially in power split hybrid vehicles, there is no kinematically decoupled device between the engine and transmission, so that vibrations transfer from the powertrain to the car body through not only engine mounts but also the driveline system. Hardware test results show that the longitudinal vibration of seat track is the most severe with a maximum peak-to-peak acceleration of 1.8m/s(2). In this study, dynamic models of the driveline system and the powertrain mounting system are respectively established in detail. The natural characteristic analyses of the two dynamic models reveal that engine order vibrations can stimulate a series of low-frequency resonances during the engine starts, which is finally embodied in the car body vibrations. Then, two dynamic models are coupled together to analyze the transient dynamic characteristics associated with engine starts. It is evident by simulations that the longitudinal vibration of car body is mainly transferred by the driveline, whereas the vertical vibration of car body is mainly transferred by the powertrain mounting system. Besides, the initial crankshaft position of the gasoline engine has an obvious effect on the engine order vibration, and further results in different transient responses of the full vehicle system.