In this paper, the mathematical models for mould-filling under counteramvity are derived on the basis of the principle of momentum. Dynamic processes of mould-filling are observed on a perspex model by means of high speed photography. The results indicate that the mould-filling under counterpmvity can be described by the second oofer nonlinear dtherential equations charucterized by damping oscillations. The velocity oscillations amplofy as the pressuring speed increases if the pressure differential functions are of the same type. On the other hand, if the functions are of different type,then the velocity oscillations depend upon whether the functions pmpressively increase or decrease. Velocity oscillations can be diminished dmmatically by increasing the local damping within the gating system. In this paper, the mathematical models for mold-filling under countevity are derived on the basis of the principle of momentum. The processes of mold-filling are observed on a perspex model by means of high speed photography. The results indicate that the mold- filling under counterpmvity can be described by the second oofer nonlinear dtherential equations charucterized by damping oscillations. the velocity oscillations amplofy as the pressuring speed increases if the pressure differential functions are the same type. On the other hand, if the functions are of different type then the velocity oscillations depend upon whether the functions pmpressively increase or decrease. Velocity oscillations can be diminished dmmatically by increasing the local damping within the gating system.