Low rank coal is usually characterized by high water content and low heating value. In order to decrease the water content and to increase the calorific value, coal need to be treatment before the use. There are many coal upgrading technologies, utilize steam combine with pressure, with the advantages and disadvantages of each. Flash drying technique provide an alternative technology to improve coal quality by utilizing low temperature hot air and low pressure that reduce the operational costs. Experiments carried out using the flash drying technique can significantly reduce the coal water content, but the water content in the product changes according to environmental conditions.