An analytical and experimental study of the transient poll behavior of polymer melt in injection molding is presented. The polymer volume flow rate into a rectangular mold cavity during the filling stage Mas determined using the Reynolds transport theory. The analysis, which neglects melt elasticity: shows that the flow rate is transient during the initial filling of the cavity. Moreover, the cavity flow rate was found to depend on material compressibility and expansivity: total volume upstream of the gate as well as rime rate of change of melt pressure and temperature. Experimental results obtained through direct flow visualization are in good agreement with the theory. The effect of gate size on the cavity flow rate was also examined. A small gate gives rise to a greater degree of melt compressibility, in the runner system. The prediction of actual flow rates into the cavity not only, enhances determination of the interventions of flow rate, morphology and properties in injection molding but also aids in the avoidance of molding defects. The results of this study can be used to improve the accuracy of CAE analysis.