This study, for the first time, revealed δ13CPOM and δ15NPOM in a wide area of the southern SOJ. Previous studies on δ13CPOM and δ15NPOM were confined to the coastal areas of the southern SOJ (Antonio et al., 2012; Nakamura et al., 2022) and sinking particles (Nakanishi and Minagawa, 2003). Seasonality was demonstrated by the δ13C and δ15N values of sinking organic particles in the deep layer (> 500 m depth) (Nakanishi and Minagawa, 2003). Significant seasonality of δ13CPOM and δ15NPOM was observed on the surface in our study, although the pattern of δ13C differed from that seen by Nakanishi and Minagawa (2003). In Nakanishi and Minagawa (2003), the δ13C of sinking particles increased with the bloom period. However, in our study, the δ13CPOM decreased along the SI-line. On the contrary, the seasonality of δ15N of the sinking particles and δ15NPOM was comparable. We were unable to determine why the seasonality of δ13CPOM of our study differed from that of Nakanishi and Minagawa (2003). However, our observations indicated that the δ13CPOM and δ15NPOM values at 100-m depth, below the euphotic layer, were significantly different from those in the euphotic zone (Fig. 2), and monthly variations were not significant below the mixed layer. Thus, the characteristics of the sinking particles in the SOJ may be different from those of POM in the surface layer.