Model development of a serial hybrid powertrain for the city bus to be used in urban transport in Slavonia Plain is presented. Data analysis of the driving speed, fuel consumption and engine speed using energy flow method in driving cycles of observed buses provided basic design parameters to the powertrain concept of serial hybrid drive. Solution concept of the diesel engine power reduction due to the optimum operation and kinetic energy recovery system ensures the project's sustainability and the return of investment in a relatively short period of time. Serial hybrid powertrain concepts in buses through energy efficiency ensure benefit in fuel economy, reduced noise and CO2 emissions versus larger total investment costs.