The system of gas penetrations through melt at symmetrical rib channels is found unstable. By varying cross-sectional area of gas channel along the flow length, this research is devoted to observing the effect of the gas penetrations system in tapering symmetrical ribs during gas-assisted injection molding. With decreasing the cross section along the flow length and decelerate the penetration of gas-tip fronts, the degree of uneven gas penetrations can be reduced and stability is improved.