This work deals with how to improve the energy efficiency, the safety, reliability and the driving comfort by powertrain control in EVs and HEVs from three aspects: 1) How to improve the energy efficiency: Using two cases study in Chapter 2 to present the idea about how to improve the powertrain efficiency of EVs and HEVs. On the other hand, the simulation study shows the current solution in EVs of increasing the driving range by enlarging battery pack size is at the expense of EVs energy efficiency, not to mention the increasing cost and the increasing difficulty of battery management system (BMS); 2) How to improve the safety and reliability by BMS: Battery is the most expensive component both in pure electric and hybrid powertrain. People expect the battery as a green energy solution to liberate society from the dependency of fossil fuel. The reality is that battery has many limitations and battery performance strongly depends on how the batteries are used and also on the environmental conditions. BMS should be designed to keep the battery within a safe operating window and to ensure a long cycle life based on battery limitations and performance characteristics. The popular used Thevenin battery model is analyzed to be linear and to be an improper model for state of charge (SOC) estimation. The nonlinear dynamic battery model (developed by Prof. Szumanwoski in 1980's) is used to develop the Li-ion battery model in numerical way. Finally an improved algorithm for battery SOC estimation is proposed in Chapter 3. 3) How to improve the driving comfort and reliability: a design methodology based on the co-design of scheduling and control is proposed in Chapter 4. Both the simulation and HIL test results show the method can effectively deal with the problem resulted from network-induced delays and network congestion, and can ensure the reliable and dependable control system for electric powertrain system in EVs and HEVs. This work also shows that the powertrain design and its control for EVs and HEVs are highly multidisciplinary, which requires researchers and engineers to have multidisciplinary knowledge or to cooperate closely. When people from different disciplines try to understand basic problems from other disciplines and work together closely, they may easily find the reasons and solutions to the problems. Nowadays EVs and HEVs are just at the beginning of mass production. Some unreliable driving phenomena have reported to appear during EVs operating, engineers haven't found the reason yet. As the number of EVs and HEVs entering into the market increases, more and more technology challenges will appear. For researchers it is also very important to work together with vehicle engineers to find the real reason of unreliable driving phenomena, and to use the research results to resolve the problems. Currently the auto industry is undergoing a radical transformation to phase out conventional vehicles (CVs) powered solely by internal combustion engines (ICEs.) Opportunities and challenges exist both for the auto industry and for auto research institutions. The Author believes that the trend for future vehicle powertrain will be all-electric and hybrid, and the current powertrain technology for EVs and HEVs has many aspects and potential to improve. In next decades, the development of powertrain for EVs and HEVs will focus on how to increase the energy efficiency, improve the safety, reliability and driving comfort, at the same time to make the powertrain more compact.