A common problem encountered in gas-assisted injection molding (GAIM) is "gas fingering," i.e., gas bubbles penetrating outside designed gas channels and forming finger-shaped branches. Severe gas fingering can result in significant reductions in part stiffness. In this paper, nondestructive techniques have been used to quantitatively characterize gas bubble geometry in specially designed GAIM parts. Based on the quantitative measurements, the most crucial process parameters to control gas fingering have been identified through factorial design experiments. More detailed process studies reveal that there is an intrinsic relationship between gas fingering and cross-sectional gas bubble shape. By examining the effects of gas channel design and material type on the gas bubble shape, some useful gas channel design guidelines have been obtained.