The subduction history of the northwestern Pacific remains controversial. Small-scale heterogeneities in the lower mantle near subduction zone, commonly interpreted as fragments of subducted oceanic crust, can provide key information for understanding ancient subduction and the related dynamics processes. Here, with station networks in Europe, China, and the USA, we use S-wave to P-wave conversions to map detailed scatterers locations at depths of 750 to 1600 km beneath the Sea of Okhotsk. Waveform modeling results suggest the existence of a subhorizontal heterogeneity at a depth of 880 km with a thickness of 9-16 km, a length of 300 km and an S-wave velocity decrease of 2.3-3.7% to peridotitic mantle, agreeing with oceanic crust undergoing post-stishovite phase transition. These scatterers distribute continuously along a southeastward dipping belt. Two explanations are proposed for the origin of these scatterers: (1) oceanic crust arising from southward dipping intra-oceanic Izanagi subduction during the Mesozoic, (2) anomalies entrained by the Pacific slab.