Mangrove forests on Pohnpei Island, FSM, have extremely large carbon stocks in both their aboveground biomasses and their forest substrata compared with those of other terrestrial ecosystems. A thick organic peat layer contributes to large carbon stocks in these mangrove forests. However, there is still little understand of carbon accumulation processes in peats during mangrove litter humification. In this study, we clarified the changes in organic composition of mangrove litters and also which compounds contributed to the large carbon stocks in mangrove peat by applying the ¹³C nuclear magnetic resonance spectroscopy for various types of decomposing mangrove litters. Remaining masses of mangrove litters decreased with incubation time. Especially, those of leaf litter, for which more than 90% of initial masses were lost in the first year. HB/HI (degree of hydrophobicity of organic matter) of mangrove litters, which was calculated from the ratio of hydrophobic C to hydrophilic C in decomposing litter, generally increased with litter decomposition. However, increment degrees were varied among litter types. This suggests that humification processes were probably being affected by litter quality and therefore differed among the litter types.