•Deep earthquakes are relocated precisely using both Chinese and Japanese stations.•Differential travel-time residuals are used to study the fine slab structure.•A metastable olivine wedge (MOW) does exist within the Pacific slab.•P-wave velocity of the MOW is 7–9% lower than that of the iasp91 model. Seismic tomography and numerical simulations show that the western Pacific slab bends horizontally when it reaches the boundary between the upper mantle and lower mantle beneath northeast Asia. It is expected that a metastable olivine wedge (MOW) exists in the cold core of the slab because of a delayed phase transition from olivine to its high-pressure polymorphs. However, it is still debated whether the MOW actually exists or not, and even if it exists, its physical properties, such as seismic velocity and density, are still unclear. In this work we use high-quality arrival-time data of 17 deep earthquakes occurring within the Pacific slab under northeast Asia to study the detailed structure of the slab. The deep earthquakes are relocated precisely by applying a modified double-difference location method to arrival-time data recorded at both Chinese and Japanese stations. Based on the precise hypocentral locations, a forward modeling method and differential travel-time residuals data are used to estimate seismic velocity within the deep source zone, which can decrease or remove the influence of ambient velocity heterogeneities. Our results show that the MOW does exist within the Pacific slab under northeast Asia, and the MOW has a mean velocity anomaly of 7–9% lower than the iasp91 Earth model. The existence of MOW in the slab has important geodynamic implications. It can reduce the speed of slab subduction and affect the generation of deep earthquakes.