The root system distribution of mangroves can help mangrove plants adapt to the muddy conditions of the intertidal zone and provide the ecosystem services of trapping sediments and erosion prevention. In past decades, much research has been undertaken on mangroves; however, few studies have focused on mangrove root distribution. Sonneratia apetala and Avicennia marina were the main species used for mangrove restoration in China. Until now there was still a lack of information on their root distribution and it is still difficult to address the following questions:1) What are characteristics of root distribution in S. apetala and A. marina at the individual plant level? 2) How does distribution of these characteristics vary between S. apetala and A. marina with increasing tree age at the individual plant level? 3) What are the implications, based on these root distribution features in S. apetala and A. marina that can provide a further understanding of sediment-trapping functions? In this study, we used a chronosequence design (i.e., planting of different age mangroves) in Qinzhou and Guangxi, China. Guangxi had the largest area of mangrove plantations across China and Qinzhou had the largest area plantations of S. apetala within Guangxi. We selected four kinds of monoculture forests:10-year-old S. apetala community (10Sa), 10-year-old A. marina community (10Am), 6-year-old S. apetala community (6Sa), and 6-year-old A. marina community (6Am). We used horizontal plane and trench methods to examine aerial roots and underground roots in each community. Our results showed that the range of root horizontal distribution and vertical depth in soil increased with increasing tree age. The average value of horizontal radius was 30.6 m for 10Sa, 3.85 m for 10Am, 9.74 m for 6Sa, and 2.23 m for 6Am, respectively. There were significant differences in the radius value between 10Sa and 10Am, as well as between 6Sa and 6Am (P < 0.01). The distribution diameter range of aerial roots of 10Sa, 10Am, 6Sa, and 6Am was 11.29, 4.40, 5.60, and 3.88 times that of the crown width. The vertical root distribution depth was 40 cm for both 6-year-old S. apetala and A. marina and 60 cm for both 10-year-old S. apetala and A. marina, respectively. There were no significant differences (P > 0.05) in vertical depth between S. apetala and A. marina. The density, height, and basal diameter of pneumatophores decreased from inside the crown to outside the crown, whereas these increased with tree age regardless of tree species and planting age. There was no significant difference in pneumatophore density (P > 0.05) under the crown between S. apetala and A. marina. There was no significant difference (P > 0.05) in height or basal diameter of pneumatophores within double distance of crown size between S. apetala and A. marina. Additionally, the majority of underground roots for all ages communities were distributed on the top soil (0-20 cm in depth) regardless of tree species and planting age, where 76.3% root for 10Sa, 77.6% root for 10Am, 91.9% root for 6Sa, 91.6% root for 6Am were distributed in this top soil, respectively. These results advance our information of the root system of S. apetala and A. marina, and provide new implications to our understanding of sediment-trapping functions of mangrove plants.