Abstract: Located at the front edge of the Wadati-Benioff zone of subducted northwestern Pacific plate, the velocity structure in the mantle transition zone beneath Northeastern China plays an important role in constraining the behavior of subducted slabs and material or energy exchange in mantle. By modeling P and SH triplicate waveforms from two deep events, recorded by a dense regional network stations, we show that a layer of fast anomaly, about 130 km thick, lies above the 660-km discontinuity with slight depression, which may be associated with the westward extension of the stagnant Pacific slab. The mantle transition zone, as a whole, is featured by low shear wave velocity and high vP/vS ratio, which may indicate the existence of a water-bearing mantle transition zone.