Rehabilitation of post-mine land in open-pit coal mines is necessary for sustainable development of coal. Although topsoil containing much nutrition should be secured for plant growth, there was the shortage and the erosion of topsoil by heavy rain in the tropical climate, especially in South East Asian countries. Therefore, in this study, the application of Fly Ash (FA) to topsoil was evaluated in terms of the growth of plants with Acacia mangium, aiming at securing topsoil in open-pit coal mines. As the result of the plant experiment, the inhibition of plant growth attributing to the dissolution of Aluminum (Al) from FA was observed. The growth rate of the plants was decreased at the mixing ratio of more than 40% of FA. Thus, topsoil can be secured by mixing FA in topsoil in consideration of the mixing ratio, contributing to the successful rehabilitation of post-mine land in open-pit coal mine.