Batteries are often used as energy storage in hybrid and electric vehicles. Despite of the development, technical characteristics of batteries are the most limiting factor for further increasing the energy conversion efficiency and pure electric operation of these vehicles. In this research, dual-source energy storage system, a battery pack and ultracapacitors, has been studied by simulation of hybrid city bus models. These two different types of energy storages have complementary characteristics therefore their combination provides both high energy and high power to weight ratio. Based on the simulation results, the benefits of using dual-source energy storage in hybrid city bus are evaluated demonstrating the advantages of the proposed energy storage system.