Mangroves are usually characterized by their high potential for long-term sequestration of organic carbon. Sediment accretion by these valuable habitats increased their capability of organic carbon sequestration. In this study, we aimed to examine the organic carbon sequestration of mangrove expansion (Avicennia germinans) in southern Iran. Different variables have been investigated in the sampled sediments, including δ13C, δ 15N, TOC, TN, and C/N ratio. Our findings indicated of the significant variation of measured variables in the several top interval, reflecting the existence of autochthonous and allochthonous organic substances. Our data showed that the minority of δ13C values are lower than -23% in the S3 site, suggesting that mangrove litter is a critical input OC (δ13C, -28 to -30%).On the other hand, the significantly higher value of δ13C (-14 to -23%) indicates that phytoplankton and possibly local microphytobenthos are the primary sources of organic materials inputs. Moreover, site S4 presents the highest mean value of C/N, TN, and δ15N which can be justified because of its proximity to the estuary and supply of organic matters imported from algal sources. The substantial role of mangrove forests depositing organic carbon should be investigated in the global carbon cycle.