A scalable Graphics User Interface Architecture for open Computer Numerical Control (GUIAC) application is presented in this paper. The GUIAC is based on Windows Presentation Foundation (WPF) and Model-View-ViewModel (MVVM) design pattern. The WPF and MVVM pattern are used to decouple the User Interface (UI) presentation and business logic. This method makes the CNC application UI presentation design and the CNC application logic implementation work in parallel for improving the development and maintenance efficiency. Using the extensible application markup language can reduce the UI presentation design workload. The GUIAC has a loosely coupled structure with three layers. The experiment results of developing the CNC application using GUIAC show that the application graphics user interface is scalable and maintained easily.