The hardware/software co-design of a high-quality real-time system relies heavily on the modeling of both the hardware and software components from three aspects: structures, functionalities, and constraints, especially the temporal constraints. However, there is not a systematic process for the elicitation and analysis of temporal constraints in hardware/software co-design. Furthermore, existing object-oriented methods provide no means for the explicit specification of system/component constraints in object models. This paper presents a systematic methodology for structured object-oriented analysis and specification of temporal constraints in hardware/software co-analysis and co-design using an extended High-Order Object-Oriented Modeling Technique (HOOMT). This methodology hierarchically elicits and analyzes the temporal constraints in hardware/software co-design based on the integration of the High-Order Object Model (HOOM) and Hierarchical Timed Automata (HTA). It helps to identify temporal constraints of hardware and software components as well as their interactions level by level. In addition, it helps trace the relationships among these constraints at multiple levels during the co-design of real-time systems. A case study from the hardware/software co-design of the simulated FACTS Power Transmission System is used to illustrate the feasibility and merits of this methodology.