A method for optimising a manufacturing process of mineral fibres, in which method - mineral melt is fed in the form of a melt jet to a fiberising apparatus, which comprises a number of rotating fiberising rotors, which are rotated by motor means, whereby the melt is fed to a first rotor of the fiberising apparatus towards an application point on the first rotor's mantle surface, - the feeding of the mineral melt to the first rotor is monitored by a monitoring means, such as a camera, in order to obtain feeding information, - a part of the mineral melt is thrown out from the mantle surface of the first rotor in the form of a drop cascade towards a mantle surface of a second rotor, - a first part of mineral melt is formed into fibres at the second rotor and is blown off from the rotor by blow-off means, and a second part of mineral melt is thrown out from the second rotor in the form of a drop cascade towards a third rotor, where at least a part of it is formed into fibres and is blown off by blow-off means, characterised in - measuring slip of at least two of the motor means of the fiberising rotors of the fiberising apparatus, which slip is a quantity indicating the difference between the motor frequency and the rotor frequency, - comparing the measured slip values to each other in order to obtain one or more melt division value(s), which melt division value gives information about how the mineral melt is distributed between at least two of the fiberising rotors in a fiberising apparatus, - using the melt division value(s) together with the feeding information for altering or adjusting at least one of the process parameters of the fiberising apparatus, whereby the application point of the melt jet on the mantle surface of the first rotor is altered based on the obtained feeding information and melt division value(s), or the rotating speed of one or more fiberising rotors and/or the blow-off speed of one or more blow-off means is altered based on the obtained feeding information and melt division value(s). - mineral melt is fed in the form of a melt jet to a fiberising apparatus, which comprises a number of rotating fiberising rotors, which are rotated by motor means, whereby the melt is fed to a first rotor of the fiberising apparatus towards an application point on the first rotor's mantle surface, - the feeding of the mineral melt to the first rotor is monitored by a monitoring means, such as a camera, in order to obtain feeding information, - a part of the mineral melt is thrown out from the mantle surface of the first rotor in the form of a drop cascade towards a mantle surface of a second rotor, - a first part of mineral melt is formed into fibres at the second rotor and is blown off from the rotor by blow-off means, and a second part of mineral melt is thrown out from the second rotor in the form of a drop cascade towards a third rotor, where at least a part of it is formed into fibres and is blown off by blow-off means, characterised in - measuring slip of at least two of the motor means of the fiberising rotors of the fiberising apparatus, which slip is a quantity indicating the difference between the motor frequency and the rotor frequency, - comparing the measured slip values to each other in order to obtain one or more melt division value(s), which melt division value gives information about how the mineral melt is distributed between at least two of the fiberising rotors in a fiberising apparatus, - using the melt division value(s) together with the feeding information for altering or adjusting at least one of the process parameters of the fiberising apparatus, whereby the application point of the melt jet on the mantle surface of the first rotor is altered based on the obtained feeding information and melt division value(s), or the rotating speed of one or more fiberising rotors and/or the blow-off speed of one or more blow-off means is altered based on the obtained feeding information and melt division value(s).