Swirl effervescent atomizer consists of two atomization mechanisms, effervescent and swirl. For a swirl-related atomizer, the air/gas core diameter was reported to be one of the factors affecting the widening of spray angle. A wider spray angle is important to provide a better spray distribution. The characteristic of gas core in an inside-out swirl effervescent atomizer was investigated as part of the study in understanding the mechanism of this type of atomization. The interaction effects between three independent parameters (i.e. swirlgenerating vane angle, gas flowrate and discharge orifice diameter) on the size and characteristics of the gas core were investigated. Water and nitrogen gas were used respectively as the working fluid and atomization gas. The high-speed shadowgraph technique was utilized to record the videos of the gas core structures. The video recordings were converted to image sequences and analyzed using an image processing software. It was concluded that a larger gas core was produced with an increasing vane angle or gas flowrate. The increasing gas flowrate tends to increase the gas core size for all cases of the vane angle. Increasing the orifice diameter tends to increase the gas core size for all cases of the gas flowrate. The interaction between the orifice diameter and swirl-generating vane angle on the size of the gas core was inconclusive.