Gas assist injection molding has become an increasingly important process in industry because of its tremendous flexibility in the design and manufacture of plastic parts. However there are some unsolved problems that confound the overall success of this technique. Unstable gas penetration in molded bifurcation parts caused by inappropriate mold/part designs and processing conditions is one of them. This report was to study the gas penetration behavior in gas assist injection molded bifurcation parts. Experiments were carried out on an 80-ton injection molding machine equipped with a high pressure nitrogen-gas injection unit. A bifurcation cavity with a part thickness of 20 mm was used for all experiments. The resin used was general purpose polystyrene to help visualize the gas penetration behavior in the parts. Various processing variables were studied in terms of their influence on the stability of gas penetration in the parts: gate position, mold layout, melt temperature, mold temperature, short shot size, gas pressure and gas injection delay time. The purpose of this study was to investigate the parameters affecting the stability of gas penetration, so that steps can be taken to optimize the gas penetration behavior. This would provide significant advantages in improving product quality.