Gas-assisted injection molding (GAIM) has proven itself to be a breakthrough in molding technology. However, the process involves additional gas-related parameters to control the molding process. The interaction between melt and gas makes the gas-assisted process complicated. To make satisfactory parts, the material, the mold, and process parameters determine the ＇＇moldability＇＇ of a specific system. The concept of ＇＇molding areas＇＇ on the critical plane can be extended from thermoplastic injection molding to GAIM. This research is devoted to the development of a proper moldability diagram for GAIM. The first part of this study employs gas-related parameters to define the molding area. The second parr is devoted to identifying the relative importance of process parameters. The results show that melt temperature and short shot are the two factors which most significantly affect the GAIM process. The third part of this study defines the moldability based on critical parameters. The application of the moldability concept is demonstrated in selecting the optimum rib geometry. These moldability studies are intended to give first guidelines for system optimization.