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Acacia plantations have been contributing to national economic and livelihood of millions people living in rural areas of Vietnam. It has been widely planted and accounted for nearly 50% areas of plantations in Vietnam. In this study, different planting densities including 1,110 trees ha-1 (3 × 3 m), 1,330 trees ha-1 (2.5 × 3 m), and 1,660 trees ha-1 (2.5 × 2.5 m) were tested for Acacia hybrid and Acacia auriculiformis. The growth parameters (diameter at breath height/DBH and stem height/H) were measured, and dry biomass was estimated for 4-year-old plantations. The results indicated that a 4-year-old plantation of A. hybrid had most massive DBH (11.3 cm) at planting density of 1,100 tree ha-1, while highest dry biomass (57.9 Mg ha-1) was observed in planting density of 1,660 trees ha-1. In case of a 4-year-old plantation of A. auriculiformis, highest DBH (10.0 cm) and highest dry biomass (50.4 Mg ha-1) at planting density of 1,330 trees ha-1 were recorded. At the planting densities of 1,110 trees ha-1 and 1,660 trees ha-1, a 4-year-old plantation of A. hybrid had significantly higher dry biomass as compared to that of a 4-year-old plantation of A. auriculiformis. However, the difference of dry biomass between 4-year-old plantations of two species at planting density of 1,330 trees ha-1 was not significant. This study concluded that to grow A. hybrid planting density of 1,660 trees ha-1 should be used, while density of 1,330 trees ha-1 is encouraged for planting A. auriculiformis for pulpwood production.