The fuzzy logic approach is used for the modelling of the layer microhardness of Fe-Mu binary alloys containing 0.94 wt.% Mn. The model was setup according to experimental results. Temperature and time were assigned as inputs and boride layer inicrohardness was designed as output. Unlike the general expectation in these kinds of physical problems in which the model is expected to be linear, our model is allowed to be nonlinear. So, in the next phase a nonlinear optimisation method, namely filled function method, can be employed to determine maximum microhardness of Fe-Mn binary alloys.