This paper describes a visual design authoring tool named VisualSpec. VisualSpec realizes a homogeneous hardware/software codesign process which means to perform both functional specification design and architectural specification design of HW/SW parts in a single tool and a common design specification language. The design specification captured with VisualSpec is always executable so that the designer can develop a virtual prototype system and simulation model for verifying functional requirements, performance and cost. VisualSpec combines top-down and bottom-up design approaches and facilitates reuse of cores and other IP components.