The modelling procedure for air path systems of diesel engines by quasi-LPV (linear parameter varying) techniques is shown. It is divided into three steps based on the physical structure of diesel engines. The final model is a quasi-LPV model with the EGR valve rate and the VGT vane position as the inputs, the boost pressure and the air mass flow as the outputs, the fuel mass flow as the disturbance and the engine speed as well as the VGT vane position as the scheduling variables. The model can be used for gain scheduled control of diesel engine air path systems.