Abstract: A system for automated pre-alignment of wafer with a diameter of 12 inches is developed.At first,the architecture of the wafer pre-alignment system is designed according to its task and flow.Next,several detection methods for pre-alignment are introduced and the least square circle fitting approach is used to position the wafer’s center.Two methods,i.e.,an edge flexion method to detect the range of wafer notch and an arc fitting method to calculate the notch base point,are proposed.Then,error analysis on the wafer pre-alignment system is presented.Finally,using the optical pre-alignment methods,the experiment with the 12-inch wafer is made.The experimental result shows that the repeatability of the wafer notch is less than 4.8 μm and the whole pre-alignment process spends 23 seconds,which satisfies the design requirements.