The oxygen isotope value (δ18O) of planktonic foraminifer shells is one of the most commonly used proxies to reconstruct the paleoenvironment. Neogloboquadrina pachyderma (sinistral) (N. pachyderma (sin.)) is a dominant foraminifer in subpolar and polar region that is used extensively for high-latitude paleoreconstruction. The present study examined seasonal variation in δ18O of N. pachyderma (sin.) using sediment trap samples collected over 3.5 years in the northwestern North Pacific Ocean. The vital offset value was about-1%, which agrees with previous plankton tow studies. Large shell individuals (180-250 μm) secrete their shells in a shallower and warmer environment than small ones (125-180 μm), and the difference in δ18O between shell size showed seasonal variation. Although large individuals calcify at a slower rate with slightly heavier δ18O than small individuals, differences between size classes mainly reflect different habitation in the water column and are related to seasonal changes in stratification. During summer, large individuals would calcify mainly at 25-35 m depth, while small individuals calcify near the pycnocline at ∼45 m. During winter, both size classes calcify at or slightly above the weak pycnocline at 45-65 m. Thus, the shell δ18O could record the oceanographic environment around pycnocline depth, and size-specific differences in δ18O reflect water column stratification. The annual flux-weighted δ18O corresponded to non-flux-weighted mean δ18O value due to the equal contribution of two flux peaks and differences between size classes also related to the stratification. It suggests that fossil δ18O data could represent the annual mean environment around pycnocline and past stratification.