Abstract: As one of widely concerned hazardous materials, radioactivity in the mangrove ecosystems is rarely studied. In this study, we presented radioactivity in surface sediments and sediment core collected from 10 locations of mangrove ecosystems nearby the Fangchenggang Nuclear Power Plant using high purity germanium γ spectrometry in order to explore the enrichment of radionuclides in the mangrove systems. The activities of ²³⁸U, ²²⁶Ra, ²²⁸Ra, and ⁴⁰K in surface sediments were in the range of 6.2～70.7 Bq/kg, 7.3～55.3 Bq/kg, 10.0～94.1 Bq/kg, and 26.5～479 Bq/kg with the mean value of (27.8±19.8) Bq/kg, (21.2±13.9) Bq/kg, (35.8±24.8) Bq/kg, (177±131) Bq/kg, respectively. The activities of ²³⁸U, ²²⁶Ra, ²²⁸Ra and ⁴⁰K in sediment core were consistent with that in surface sediment. No significant trend was observed for the vertical profile of these naturally occurring radionuclides (²³⁸U, ²²⁶Ra, ²²⁸Ra and ⁴⁰K). We found that radioactivity in the mangrove ecosystems was lower than that in Guangxi soil, China soil, global soil, other sea regions except coral reefs. The activity ratio of ²²⁶Ra/²³⁸U in surface sediments from the mangrove ecosystems was consistent with that from other sea regions, ranging from 0.5 to 1.0. However, the significant difference of the activity ratio of ²²⁶Ra/²³⁸U between the mangrove ecosystems (0.5~1.0) and coral reefs (<0.1) was determined by distinct sediment sources with geologically originating sediment in the mangrove ecosystems and biogenic carbonate sediment in coral reefs, respectively. Overall, the enrichment of radionuclides was not observed in the mangrove ecosystems in Guangxi.