Particulate organic carbon (POC) is one of the main forms of carbon and plays an important role in aquatic ecosystems and global carbon cycling. Marine and terrestrial ecosystems are the two important carbon pools for the estuaries on earth, so the investigation on the POC in estuaries is dramatically practical for the research of global carbon cycling. Generally, the POC in estuaries can be classified into two categories by its source (terrestrial input and marine organism production). Stable carbon isotope (δ¹³C) and C/N are the two recommendatory methods for measuring and analyzing the source of organic carbon, and the precision of stable carbon isotope method (δ¹³C) is better than that of C/N method because of the stability of δ¹³C. The present study aimed to investigate the distribution of POC and its sources in the Pearl River Estuary in summer through the POC measurements of 20 sites in August, 2010. The results showed that the concentrations of POC ranged from 98.5 to 1929.8 μg/L (the average of 541.9 μg/L), and the concentrations decreased with the salinity gradient in the northern and middle part of the Pearl River Estuary and then increased from the middle part to the southern part of the Pearl River Estuary, and the concentrations of POC in the bottom waters layers were higher than those in the surface water layers. The total suspended matter (TSM), chlorophyll a and hydrodynamics were the major factors that influenced the POC spatial distribution, and the δ¹³C values of TSM increased with the salinity gradient and varied from -27.05‰ to -21.03‰ (the average of -24.57‰), and the values in the bottom water layers were higher than those in the surface water layers. Our results showed that the terrestrial input and marine phytoplankton production contributed to the POC together in the Pearl River Estuary. According to the analysis of binary mixture model, most POC in the northern and middle part of the Pearl River Estuary was subject to the terrestrial organic carbon, which accounted for 50% to 88% (the average of 64%) of the total POC. In contrast, the marine organic carbon was the main contributor to the POC in the southern part of the Pearl River estuary, and it accounted for 50% to 89% (the average of 68%) of the total POC. Compared with two decades ago, the distribution and source of POC varied significantly, which is mainly due to the decrement of sediment discharge and increment of nutrient inputs into the Pearl River estuary. We predict that the sediment discharge rate of the Pearl River will decrease gradually with the construction of more large reservoirs, and then the contribution of terrestrial organic carbon to the POC in the Pearl River estuary will be decreasingly.