During ageing of Fe-Cu alloys, for standard ageing conditions, peak hardness and strength is often observed after several hours. The significant strengthening is attributed to a dense distribution of very small bcc-Cu precipitates of 2-3 nm size. Using conventional numerical precipitation kinetics models for diffusion-controlled transformations, the kinetics of strengthening cannot be consistently described. One of the issues in this aspect is the fact that, after reaching peak hardness, a strong decrease in number density is observed experimentally, which cannot be explained by classical Ostwald ripening theory. In the present study, a new methodology for simulation of the copper precipitation kinetics in the early stage is suggested. The basic idea of this approach is to take into account the composition variation of the Cu-precipitates with respect to the Fe content during the precipitation reaction. The simulation results are compared to experimental data reported in literature. Consistent agreement between experiment and simulation can be achieved with the new methodology.