This paper proposed an Application Programming Interface (API) for sketch-based user interface (UI). A recent research direction in modeling interface is to automate or assist the sketch-to-3D translation process. This trend, known as sketch-based interface for modeling, is motivated by the ease of sketching and the ability of human viewers to imbue so much meaning into a sketch. In sketch-based user interface, user expresses visual ideas or messages as freeform strokes on pen-based systems, and the computer takes appropriate action by analyzing the perceptual features of the strokes. Although sketch-based system is powerful, it came with the cost of code complexity. The two main contributors to code complexity are: pen-tablet display and the difficulties in interpreting the sketches. This paper proposed a sketch-based API to reduce the code complexity in sketch-based user interface application. The proposed sketch-based API will be tested by building a sketch-based user interface. © 2010 IEEE.