As a major constitute of ocean biological pump, the export of particulate organic carbon (POC) from the euphotic zone is important for the evaluation of ocean carbon budget. However, it is not clear the major drivers that regulate the POC export in the euphotic layer in a diurnal cycle. In this study, we collected particle samples in the upper 200m water column from South China Sea (SEATS Station) in September 2020. The diurnal variations in POC and amino acids (AA) concentrations and stable carbon isotope values were analyzed. The POC δ¹³C ranged from -26.0‰ to -21.1‰, which shows no diurnal variations. The AA δ¹³C ratios varied from -26.7‰ to -19.9‰ with different profiles observed between samples collected at 14:00 PM and 20:00 PM. A Linear regression of POC δ¹³C and carbon weighted AA δ¹³C shows varied slopes for samples collected at different times: a strong linear correlation was observed at 14:00 PM with a slope 0.94, while at 20:00 PM the slope was only 0.1. Since the AA was labile component more influenced by the newly produced OC from the primary production, the varied relationship between POC and AA δ¹³C suggests that POC export may be regulated by the decoupled variations in primary production, particle dynamics and degradation in a diurnal cycle. This study observed for the first time the diurnal variation in labile and bulk particulate organic carbon δ¹³C in the upper layer of the South China Sea basin, which helps to distinguish the factors regulating the POC export in the euphotic zone.