Characteristics of gas penetration in a spiral tube during gas-assisted injection molding were investigated experimentally. It was found that the coating melt thickness, which decreases with increasing gas pressure, is uniformly distributed behind the gas front during the primary penetration period. When the gas pressure reaches a critical value, the coating melt thickness becomes constant. Skin melt thickness also increases with decreasing melt temperature and mold temperature. Near the gas front the coating melt thickness varies significantly, and the variation shows two types of characteristics. At low injection gas pressures and longer delay times, the coating thickness at the gas front first decreases and then increases significantly, resulting in a melt-thinning region of a gas-bulge shape. This indicates the existence of a secondary fountain flow around the gas front tip. At high gas pressures and short delay times, the skin melt thickness around the gas front increases dramatically as a result of melt shrinkage during the secondary penetration stage.