<p>The trend of implementing a diversification strategy, which involves introducing two or more species, is becoming increasingly prevalent on a global scale, particularly in tropical forest plantations that were previously dominated by single-species compositions. Improving productivity and enhancing other ecosystem services are the main reasons for this conversion. While mixing effects in temperate forests and plantations are well supported by empirical data, such information is scarce for tropical forest plantations. We analyzed the quantitative aboveground tree biomass—structure—tree diversity relationship in a tropical mixed-forest plantation in Northern Vietnam, hypothesizing that aboveground tree biomass would be increased with greater structural and species diversity. The study site was in a mixed abandoned plantation of <i>Acacia mangium</i> and <i>Acacia auriculiformis</i> in Northern Vietnam. A one-hectare permanent plot was installed in which all trees were identified individually by species, measured (diameter and height), and georeferenced. The descriptive analysis found that the study area contains 110.66 tons/ha of biomass in the aboveground tree compartments. The structural and species diversity analysis indicated high richness and evenness. A set of models was fitted and ranked to determine the relationship between aboveground tree biomass and structural and specific diversity. The results indicate that the species diversity and evenness, described by the Shannon index, and height differentiation indexes were negatively significant for both species while basal area was only positively significant for <i>Acacia mangium</i>. According to the model sensitivity analysis, aboveground tree biomass—and therefore carbon concentration—decreases as species diversity increases.</p>