Abstract: The 2008 M_W7.9 Wenchuan earthquake occurred in Wenchuan, Sichuan Province caused heavy casualties and great economic losses. Using the method proposed by Shcherbakov and Turcotte, we estimate the magnitude of the largest aftershock following the Wenchuan mainshock. The results show that, when aftershock’s magnitude becomes much large, the corresponding aftershocks in the current aftershock catalogue we have used may be missed, which causes the phenomenon that the catalogue deviates from the prediction calculated by the Gutenberg-Richter relationship, and this deviation also causes the big difference between the recorded largest aftershock and inferred largest aftershock. However, using the empirical relationships between the magnitude and surface rupture length or rupture area, we find that the sub-event caused by the Guanxian-Jiangyou fault has a similar magnitude with the inferred largest aftershock. Moreover, based on the model which describes the instability time of fault frictional slip and the Brune’s source model, we demonstrate that the shear seismic wave generated by the earthquake rupture on the Beichuan-Yingxiu fault could trigger the rupture on the Guanxian-Jiangyou fault at a very short time, which finally could lead to an earthquake. Therefore, the triggered event could be considered as one of the largest aftershocks and also be involved in the aftershock’s catalogue.