Blue carbon pertains to carbon stock that is stored by marine ecosystems including mangrove forest. Density and blue carbon stock assessment was conducted covering mature Rhizophora stylosa Griff. plantations in Bohol, Philippines. Three stand types were assessed: thinned mature stand (0.34 tree m−2 at 55 years old; 35 ha); non-thinned mature stand (1.2 tree m−2 at 55 years old; 20 ha) and young stand (2.6 tree m−2 at 20 years old; 150 ha). These plantations were initially established with a plant spacing of 0.5 m × 0.5 m. Non-destructive plot sampling technique was used to account biomass and carbon stocks of trees and sediments. Five (5) sample plots measuring 200 m2 were allocated for each stand type. Results showed that the total carbon stock of plantations amassed to 64.5 ktC or 236.6 ktCO2e. More than half (at least 54%) of this stock is imbedded in sediment. Individual accounts showed that carbon stock was largest in non-thinned stand with 435.2 tC ha−1, followed by thinned stand (408.5 tC ha−1) and young stand (276.8 tC ha−1). There was no significant difference between thinned and non-thinned mature stands carbon stocks. Such finding suggested the potential of having thinning intervention. Following the self-thinning rule of 1.1 to 1.3 tree m−2, the local community can harvest as much as 423 trees ha−1 yr−1 for 35 years starting at the plantation age of 20 years. Further, the additional carbon stock between the ages 20 to 55 years could reach about 580 tCO2e ha−1 with an economic value of USD 2,962 ha−1. With this potential benefits, it is recommended that Banacon Island should pursue a carbon offset project for its plantations.