Abstract: A record of organic nitrogen isotope (δ¹⁵N_org) variations since the Late Miocene (ca. 8 Ma) is presented from sediment cores at IODP Site U1438B in NW Pacific. The record reflects the history of δ¹⁵N variations in the upper water of NW Pacific. The δ¹⁵N_org record showed a long-term increasing trend of approximately 2.3‰ since the Late Miocene. Four stages were recognized: low values in the Late Miocene (3.98‰±0.25‰), an increase during the Pliocene (3.96‰~5.43‰), relatively low values during the Pliocene-Pleistocene transition (4.66‰±0.28‰), and further sustained increase in the Quaternary (5.16‰±0.52‰). Comprehensive analysis suggested that the long-term δ¹⁵N_org increasing trend since the Late Miocene reflected mainly an increased contribution of water column denitrification (WCD) to the oceanic denitrification in the Eastern Tropical Pacific, with the signal transported to the Philippine Sea via ocean circulation. Specifically, the lower δ¹⁵N_org values in NW Pacific during the Late Miocene might reflect weaker WCD and weaker upper ocean circulation in the Eastern Pacific. The significant increase in δ¹⁵N_org during the Pliocene indicates an enhanced denitrification signal from the Eastern Pacific. The continued increase in δ¹⁵N_org during the Quaternary was related to the sustained climate cooling, significant sea-level fall, and the expansion of the Eastern Pacific Oxygen Minimum Zone. Regional nitrogen fixation might have also contributed to the evolution of nitrogen isotopes in NW Pacific, including the relatively low values during the Pliocene-Pleistocene transition (due to enhanced nitrogen fixation) and the renewed increase in the Quaternary (due to weakened nitrogen fixation). Therefore, the δ¹⁵N_org record from Site U1438B in the Philippine Sea can be considered as a "regional expression" of the broader Pacific nitrogen cycle processes. It captured major events that drove shifts in Pacific nitrogen cycle patterns (e.g., the evolution of the Eastern Tropical Pacific Oxygen Minimum Zone) and was also influenced by regional nitrogen fixation. Further research on this record could potentially reveal a more detailed evolutionary history of the Pacific nitrogen cycle during the Late Cenozoic.