Water assisted injection molding technology has received extensive attention in recent years, due to its lightweight, relatively lower resin cost per part, faster cycle time, and its flexibility in the design and manufacture of plastic parts. However there are still some unsolved problems that confound the overall success of this technology. Unstable water penetration in molded symmetric channels is one of them. This report was to study the water penetration behavior in the water assisted injection molded symmetric cavity. Experiments were carried out on an 80-ton injection-molding machine equipped with a water injection unit. A plate cavity with symmetric water channels was used. The materials used were amorphous polystyrene and semi-crystalline polypropylene. Various processing variables were studied in terms of their influence on the water penetration behavior inside the symmetric parts: melt temperature, mold temperature, melt fill speed and fill pressure, short-shot size, water temperature and water pressure, and water injection delay time and water hold time. The effect of channel layout was also investigated. Tapered channels were found to mold parts with better penetration stability than flat channels. In addition, the penetration behaviors of water and gas in molded symmetric parts were also compared. The experimental results suggested that water penetration stability is more stable than that of gas in molded symmetric parts.