Abstract: Surface rupture is most straightforward demonstration of earthquake rupture features. Detailed and exhaustive mapping of surface rupture zones is important for understanding better earthquake mechanism and dynamic rupture processes. In this paper，we document our effort in finescale mapping of the 5.12 Wenchuan earthquake surface rupture on the YingxiuBeichuan fault near Hongkou. Surface rupture in this short stretch shows clearly large alongstrike variations in both mapview geometry and coseismic slips. Based on the discontinuities in slip and geometry，the rupture can be divided into four sections，from southwest to northeast: the Shenxi Gou，the Miaoba，the Gaoyuan and the Bajiao Miao sections，respectively. The vertical offsets are larger in the Shenxi Gou and the Bajiao Miao sections，reaching locally 5～6m in maxima，and generally lower in the Miaoba section，1～2m or less. Near the Gaoyuan village，the surface rupture consists of two subparallel branches. The northern branch shows mostly rightlateral slip with minor SE sideup thrusting，and the southern one mostly SE sideup thrusting. The sense of vertical throw is thus opposite to the general NW sideup thrusting of the Wenchuan rupture. We propose that it is due to the inheritance at shallow depth of the SEdipping fault of the klippen system in the south. Together with the locally reversed fault plane near the Shaba village，Beichuan，it demonstrates that the structures of old fault zones in guiding earthquake ruptures in the updip propagation of earthquake rupture. We also discuss the cutting relationship of slickenside striations of two different plunges and their geological and seismological implications，by pointing out that the curved striations may indicate the low initial shear stress at some localities on the Yinxiu-Beichuan fault before the Wenchuan earthquake.