Abstract: The purpose of this paper is twofold: (1) To describe the perceptions of a participant-observer about a new geometry course integrating with a dynamic geometry tool, Geometer’s Sketchpad (GSP), to help elementary preservice teachers (EPSTs) prepare geometric knowledge needed for teaching. The design of the new geometry course was informed by Ball’s (2003) theory of Mathematical Knowledge Needed for Teaching. (2) To report the effects of using GSP to support EPSTs in developing the knowledge needed for teaching geometry.