In modern era, establishing mixed tree species plantations is becoming more popular in an effort to protect the environment. Reports on comparing monoculture tree plantations under High Density Plantation (HDP) is meager. Therefore, an investigation has been carried out to compare the production of total biomass, carbon content, carbon sequestration, and oxygen release between monoculture plantation - HDP 1 (1 species), mixed tree plantations of 7 species (HDP 2) and mixed tree plantations of 14 species (HDP 3). The experimental results indicated that mixed tree plantation with 14 different tree species (HDP 3) has sequestered the highest carbon (4863.03 kg per ha per year) and oxygen release (12968.98 kg per ha per year) while the least was recorded in other two plantations. Hence, the present study strongly recommends that trees grown under mixed plantations with more number of species encompassing diverse tree species plays a superior role in enhancing the environmental quality compared to monoculture plantation.