Abstract: This study determined the boundary between southern and northern segments of the Wenchuan earthquake fault based on the analysis of stress field after the main shock. The method of dividing fault or plate boundary into sections by means of stress field uniformity analysis is utilized. Aftershock focal mechanisms were obtained by using ldquo;cut and pasterdquo; (CAP) method, and used to infer the stress field. The results show that the boundary between the northern and southern segments of the seismogenic fault is located nearBeichuanCounty, similar to those from previous studies. The inverted aftershock stress results show that the orientation of maximum principal stress axes for both northern and southern segments is ENE-WSW, and nearly horizontal. The average misfit and confidence level range of stress inversion result are relatively large, probably due to the stress heterogeneity in the northern and southern segments. Since this research only gives a gross segmentation of the seismogenic fault based on focal mechanism determination of large aftershocks, a further division of the northern and southern segments should be studied in detail in future.