Nowadays hybrid power trains represent one of the most effective ways of reducing fuel consumption and exhaust gas emissions in vehicles. Electric hybrid power trains has been in focus of hybrid technology since 1970 but instinctive limitations of electric systems like low round trip efficiency and low power density motivate some researchers to search for alternative systems in power train hybridization. Storing energy in a high-speed rotating wheel is one of them. Despite of its benefits like high power density and large round trip efficiency, it has some complexities. One of them is to maintaining continuous power flow between three rotating shaft with independent varying speed. In this thesis, an innovative parallel structure of flywheel hybrid power train is introduced which consists of a high speed flywheel, a fixed ratio gear set, a clutch, an infinitely variable transmission (IVT) and a torque coupling. Reducing energy dissipation by disengaging the flywheel clutch in flywheel standby mode and maintaining power flow in flywheel power train with its slipping mode are some privileges of the represented structure. It is shown that an IVT structure with proposed power flow represent better efficiency than other types. Using a fixed ratio gear set before the flywheel reduces power loss in IVT. Mechanical behaviors of all consisting components are modeled considering the effect of inertia, aerodynamic drag and friction. This model is integrated with a conventional power train in ADVISOR software and is controlled by a combined control strategy consisting operating point optimization, regenerative braking and automatic engine start-stop. The Simulation results of the system in standard drive cycles for a light car shows reduction of about 25 percent in fuel consumption and 30 percent in exhaust gas emissions. The ratio of the flywheel gear set has been optimized. A further investigation about comparing flywheel hybrid power trains with electrical ones is also carried on which shows that a mechanical power train represent better efficiency in low power density vehicles like city buses. The results also indicate that mechanical hybrid power trains have better efficiency in high power density vehicles like light vehicles. In conclusion, simulating a city bus in city bus drive cycles shows about 30 percent reduction in fuel consumption even without start-stop mode which confirms that mechanical power trains are more efficient in heavy vehicles. Key Words: Mechanical Hybrid Vehicles, Flywheel hybrid, Fuel Consumption Reduction, Exhaust Gas Emission Reduction