Seawater samples were collected at discrete depths from five stations across the polar front in the Drake Passage (Antarctic Ocean) by the $20^{th}$ Korea Antarctic Research Program in December, 2006. Nitrate concentrations of seawater increase with depth within the photic zone above the depth of Upper Circumpolar Deep Water (UCDW). In contrast, ${delta}^{15}N$ values of seawater nitrate decrease with depth, showing a mirror image to the nitrate variation. Such a distinct vertical variation is mainly attributed to the degree of nitrate assimilation by phytoplankton as well as organic matter degradation of sinking particles within the surface layer. The preferential $^{14}{NO_3}^-$ assimilation by the phytoplankton causes $^{15}{NO_3}^-$ concentration to become high in a closedsystem surface-water environment during the primary production, whereas more $^{14}{NO_3}^-$ is added to the seawater during the degradation of sinking organic particles. The water-mass mixing seems to play an important role in the alteration of ${delta}^{15}N$ values in the deep layer below the UCDW. Across the polar front, nitrate concentrations of surface seawater decrease and corresponding ${delta}^{15}N$ values increase northward, which is likely due to the degree of nitrate utilization during the primary production. Based on the Rayleigh model, the calculated ${varepsilon}$ (isotope effect of nitrate uptake) values between 4.0%o and 5.8%o were validated by the previously reported data, although the preformed ${delta}^{15}{{NO_3}^-}_{initial}$ value of UCDW is important in the calculation of ${varepsilon}$ values.