Growth response of Acacia mangium seedlings to cowdung and NPK fertilizers was studied in Bangladesh Forest Research Institute, Chittagong. Use of cowdung and chemical fertilizers either individually or combinedly caused significant increase in seedlings’ height and biomass production. Application of cowdung alone in soil and soil-cowdung mixture at 3:1 ratio showed best performance to enhance both height increment and biomass production. The height growth and biomass production of A. mangium seedling ranked second when soil – cowdung mixture at the rate of 5 : 1 ratio was applied in combination with NP each at 60 ppm level. Chemical fertilizer alone showed less effect on the growth of the seedlings.