Three dimensional CAD data viewer helps a user to view and use many different types of CAD data without having costly programs that produced them loaded on their computers. A viewer for standard data formats such as STEP and IGES is more useful since most of the CAD systems provide translators for them. We developed a 3D CAD viewer for STEP AP203 solid and assembly data. In addition to the standard shading and assembly tree display, functions for 3D markups and measurement of distance and angles were implemented.