Downward export fluxes of particulate organic carbon (POC) transported to the interior of the ocean is an important index to evaluate the efficiency of BCP and one of the core parameters of the marine carbon cycle, the constraint of which remains however a daunting challenge. This study examines spatial patterns of particle size distributions (PSDs) and carbon flux within the upper 1000 m of the oligotrophic North Pacific Ocean based on in situ Underwater Vision Profiler 5 (UVP5) measurements and simultaneous ²³⁴Th samplings. We established an empirical relationship between particle size structure and POC flux applicable to the Northwest Pacific Ocean and obtain carbon flux results with high vertical resolution. The POC export fluxes averaged at 3.20 ± 2.54 mmol C m⁻² d⁻¹ in Kuroshio Extension (KE) were significantly higher than fluxes in North Pacific Subtropical Gyre (NPSG, averaged at 1.22 ± 0.58 mmol C m⁻² d⁻¹). Furthermore, our results revealed a positive correlation between POC export fluxes at the euphotic zone base and the attenuation rate of POC fluxes in twilight zone, higher Marin curve b values (averaged at 0.44 ± 0.10) exist in KE than lower latitudes. By analyzing the variation of the attenuation rate in twilight zone, we concluded that the vertical structure of the b value was highly corresponding to the PSD slope, with high b values generally characterized by flatter PSD slope, that is, a high relative abundance of large versus small, indicating that the particle size distribution profoundly affects the POC export and remineralization process.