Abstract: Loss coefficient and block coefficient are interrelated in fact and the relation between them represents the relation between loss and separation.An engineering model is established to connect them,based on an analysis of many fan designs.The model determines the block coefficient from the loss coefficient,because the block coefficient derived from test loss coefficient is more believable than that from experience.At the same time,a computation test of a transonic fan is given as an example to verify the feasibility of the model.A conclusion has been drawn from the computation results that this model can estimate exactly the block coefficient and flow direction at the trail edge of rotor so as to improve the situation of the stator.It also explains why the rotor efficiency is superior to the stage efficiency.