Abstract: The 12th May 2008 Wenchuan earthquake is one of the very few reported continental oblique reverse-faulting events with crustal-scale co-seismic slip partitioning on parallel reverse faults. Between the two principal NE-striking ruptures, the NW-striking Xiaoyudong also broke during the quake. This paper reports a detailed mapping and coseismic slip measurements along the Xiaoyudong rupture. The generally N55°W-striking Xiaoyudong rupture extends for ~8km and accommodates a left-lateral and SW-side up oblique motion. In the map view, the rupture zone has a relatively stable strike over the most part of the fault, but changes rapidly at its southeast end, where the strike changes from N55~65°W to nearly north-south southward, and then connects with the Cifeng section of the Pengguan fault. Fault scarp is highest in the northwest part, reaching 3.4m, and decreases southeastward to less than 0.2m at the southeastern end, with the average slip gradient of 6×10-3 along the strike. Left-lateral offset measurements are sparser than the vertical, and are concentrated in the middle reach near Xiaoyudong Town. The maximum observed left-lateral displacement is 2.2m. Left-lateral offsets co-vary with the vertical component. The near-surface fault has low to medium dip angle, being 30°±15°. Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a ~30°SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kinematically, the Xiaoyudong fault likely functions as a tear and conjugate fault, connecting and transferring coseismic slip transfer between the Beichuan and Pengguan faults. Xiaoyudong rupture is a tear fault with the geometry of oblique ramp, adjusting the lateral variations of the northeast-trending main fault kinematics. Slip vector on the Xiaoyudong fault is compatible with the regional N70° and 80°E principal compressional stress direction, oblique to the current mountain front orientation.