A method of modelling at least one property of paper, paperboard or board which is an end product in connection with use of a paper machine, the method comprising: taking a sample from liquid fibre pulp used for manufacturing the end product, wherein the sample is taken from the liquid fibre pulp which is to be conveyed into a headbox of the paper machine or has proceeded into it; arranging the particles of the sample according to the particle size by use of a fractionator utilizing hydrodynamic flow resistance of particles; determining, at a moment of time T1, a distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes fractionated by the fractionator; determining, at a moment of time T2 which is later than the moment of time T1, at least one property of the end product; generating a model based on the determined at least one property of the end product and the distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes, wherein the model connects the property of the end product received at the moment of time T2 to a distribution of the property of the sample of the liquid fibre pulp received at the moment of time T1; taking a sample from the liquid fibre pulp used for manufacturing the end product; arranging the particles of the sample according to the particle size by use of the fractionator utilizing hydrodynamic flow resistance of particles; determining, at a moment of time T3 which is later than the moment of time T2, a distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes fractionated by the fractionator; and modelling, at a moment of time T4 which is later than the moment of time T3, the at least one property of the end product on the basis of the generated model when the distribution of at least one property of the sample at the moment of time T3 is known. taking a sample from liquid fibre pulp used for manufacturing the end product, wherein the sample is taken from the liquid fibre pulp which is to be conveyed into a headbox of the paper machine or has proceeded into it; arranging the particles of the sample according to the particle size by use of a fractionator utilizing hydrodynamic flow resistance of particles; determining, at a moment of time T1, a distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes fractionated by the fractionator; determining, at a moment of time T2 which is later than the moment of time T1, at least one property of the end product; generating a model based on the determined at least one property of the end product and the distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes, wherein the model connects the property of the end product received at the moment of time T2 to a distribution of the property of the sample of the liquid fibre pulp received at the moment of time T1; taking a sample from the liquid fibre pulp used for manufacturing the end product; arranging the particles of the sample according to the particle size by use of the fractionator utilizing hydrodynamic flow resistance of particles; determining, at a moment of time T3 which is later than the moment of time T2, a distribution of at least one property of the sample of the liquid fibre pulp as a function of at least two different particle sizes fractionated by the fractionator; and modelling, at a moment of time T4 which is later than the moment of time T3, the at least one property of the end product on the basis of the generated model when the distribution of at least one property of the sample at the moment of time T3 is known.