A method for monitoring the performance of a subsystem or function of a forest machine, comprising the steps of: performing processing of several tree trunks by a harvester head connected to the forest machine; during the processing of each trunk, taking measurements to find out a trunk size class specific to the trunk being processed; collecting, during the processing of said trunk, measurement data associated with said trunk and relating to the function of a subsystem or the performing of a function; continually collecting said measurement data for several trunks during the processing of said several trunks; characterized in that the method further comprises the steps of: making a division of said collected measurement data into several trunk size classes to compensate for processing conditions, wherein the division is based on the trunk size class of the trunk said collected measurement data is associated with, and wherein the trunk size class is based on a diameter of the trunk, or a volume of the trunk, or a remaining volume of the trunk; computing at least one characteristic value for said several trunks, wherein the computation takes place on the basis of the measurement data divided into the trunk size classes, to describe the performance of said subsystem orfunction; repeatedly computing the characteristic value; and storing the time history of the computed characteristic value in the form of a performance history of the forest machine, by means of which the variation in time of the computed characteristic value and thereby the performance of the subsystem or function can be monitored. performing processing of several tree trunks by a harvester head connected to the forest machine; during the processing of each trunk, taking measurements to find out a trunk size class specific to the trunk being processed; collecting, during the processing of said trunk, measurement data associated with said trunk and relating to the function of a subsystem or the performing of a function; continually collecting said measurement data for several trunks during the processing of said several trunks; characterized in that the method further comprises the steps of: making a division of said collected measurement data into several trunk size classes to compensate for processing conditions, wherein the division is based on the trunk size class of the trunk said collected measurement data is associated with, and wherein the trunk size class is based on a diameter of the trunk, or a volume of the trunk, or a remaining volume of the trunk; computing at least one characteristic value for said several trunks, wherein the computation takes place on the basis of the measurement data divided into the trunk size classes, to describe the performance of said subsystem orfunction; repeatedly computing the characteristic value; and storing the time history of the computed characteristic value in the form of a performance history of the forest machine, by means of which the variation in time of the computed characteristic value and thereby the performance of the subsystem or function can be monitored. making a division of said collected measurement data into several trunk size classes to compensate for processing conditions, wherein the division is based on the trunk size class of the trunk said collected measurement data is associated with, and wherein the trunk size class is based on a diameter of the trunk, or a volume of the trunk, or a remaining volume of the trunk; computing at least one characteristic value for said several trunks, wherein the computation takes place on the basis of the measurement data divided into the trunk size classes, to describe the performance of said subsystem orfunction; repeatedly computing the characteristic value; and storing the time history of the computed characteristic value in the form of a performance history of the forest machine, by means of which the variation in time of the computed characteristic value and thereby the performance of the subsystem or function can be monitored.