Bio fuel plays an important role in the energy production of the forest industry. The initial moisture of the fuel typically varies between 50 - 63 w-%. A multi stage drying concept has been developed for the drying of wet bio fuel. To adjust the temperature levels in the multi stage drying system there must be a simple and fast method to define the fuel initial moisture. A method to define the initial moisture based on the measurement of the outlet air moisture was experimentally studied. Regular shaped wood particles having different moisture content were dried in a fixed bed reactor. The difference between outlet and inlet air moisture was measured. According to results, it is possible to make conclusion about the initial moisture of the material by measuring the moisture of the outlet air.