There are several approaches to the hardware/software design in embedded systems, ranging from the traditional sequential methods which focus on the determination of the hardware architecture prior to software design, to newer object-oriented approaches that attempt to apply software engineering methods to hardware design without a systematic process. This paper discusses a structured object-oriented methodology for the integrated co-analysis and co-design of hardware/software systems using an extended High Order Object-oriented Modeling Technique (HOOMT). This methodology offers a uniform method for hardware and software developers to jointly develop the specifications for and partitioning of the hardware and software components of a system, as well as developing the interfaces between components, and allows easy design migration of components between hardware and software. In this paper it is applied to the co-analysis/co-design of the hardware and software of a simulated advanced power grid control system.