The gating system is the technological basis of Mg alloy in expendable pattern casting （EPC） under controllable pressure. The too large or small gating size can affect pattern

filling rate and casting quality.Filling rate is dominated by gate size lower than critical value while it is mainly controlled by evaporazation rate of the foam pattern and transmission velocity of thermal degradation products with gate size higher than critical value in conventional EPC. Based on four sets of samples

filling rate change of liquid Mg alloy with different gate area is studied. An approach to the relationship between critical gate area and pattern size under controllable pressure was conducted. The results showed that there exisits a larger critical gate area of Mg alloy with faster filling rate in EPC under controllable pressure of both compressed air and vacuum than under gravity pressure

increasing with casting modulus increase. The results will be helpful to the gating system design of Mg alloy under controllable pressure in EPC.