Abstract: A method for controlling a power source (1, 10) of a forestry machine, wherein the power source utilizes directly or indirectly one or more work devices and/or handling devices (3, 4, 7, 8) positioned in the forestry machine and is controlled by a control unit (2). When the control unit in the forestry machine gives one or more work or handling devices a control command starting a change operation, it is transmitted substantially simultaneously to the power source. When arriving at the power source, one or more control commands requiring a power change of the power source affect control parameters of the power source, predicting the future load of the power source.