We analyzed N2O isotopomer ratios (distribution of isotopes within N2O molecules) in the eastern tropical North Pacific. The N2O isotopomer ratios indicate the contribution of denitrification in the oxygen minimum zone (OMZ, ~600 m in depth) in the western North Pacific, which is not consistent with the widely accepted nitrification hypothesis. Our models indicate that the N2O yield per mole O2 consumed (dN2O/-dO2) is 0.008 (0--0.015) nmol/¿mol during remineralization and nitrification in the western North Pacific. Nitrification in aerobic deep waters is a minor source of oceanic N2O, whereas the N2O production in the OMZ is the dominant factor for the oceanic N2O distribution. The denitrification in the OMZ is consistent with the correlation between ΔN2O (level above atmospheric equilibrium) and AOU (apparent oxygen utilization), and the parallel 18O-enrichment of N2O and O2 in the OMZ, which have been believed to support the nitrification hypothesis.