This paper deals with a predictive model of kinematical performances in 5- axis milling within the context of HSM. Capacities of each axis as well as some NC unit functions can be expressed as limiting constraints. The proposed model relies on each axis' displacement in the joint space of the machine-tool and predicts the most limiting axis for each trajectory segment. Thus, the calculation of the relative feed rate tool- surface can be performed highlighting zones for which the programmed feed rate is not reached and so, it constitutes an indicator for trajectory optimization. The efficiency of the model is illustrated through an example.