From the March/Micra released in 2002 to the LEAF EV today, on-board electronic systems have come to possess bigger software programs and more complexity in the transition to distributed high-speed ECU network systems. Meanwhile, faster time-to-market requirements have shortened the development period. If a conventional development process like the waterfall model is applied to develop a distributed ECU network system, numerous problems may occur at the vehicle testing stage that requires a lot of rework. To avoid that, we devised a new development methodology of distributed ECU systems and implemented it in our vehicle function development programs to improve development work quality across the entire spectrum of vehicle electronics.This development methodology consists of two principal elements. One is an Electronic Integration Platform (EIPF) and the other one is a Design-EIPF (D-EIPF). EIPF is a system-testing environment for actual vehicle parts and D-EIPF is a system evaluation environment for virtual vehicle parts. Both of them are based on Computer-Aided Engineering (CAE) and use Model Based Development (MBD) simulation technologies.First, in the design phase, we create simulation models from the control specifications using Statemate and/or Simulink/Stateflow and run simulations to check the behavior of individual ECUs alone. Second, we create the D-EIPF environment composed of the ECU models and communication network models. The latter models run on CANoe using multiplex communication and an in-house tool for the harness connections. D-EIPF enables system-level testing as well as customer aspect testing at an early phase before EIPF tests and in-vehicle testing using actual parts. This means that we can validate system-level functions before building any bench test system or prototype vehicle. In addition, D-EIPF and EIPF use a common test suite that can run the same test scenarios between them, which improves traceability and development work efficiency.This paper gives a general description of the life cycle process using the EIPF and D-EIPF system, which is a symmetrical process for obtaining strong solutions.