Steam-based extrusion (SBX) is widely utilized for starch-based products but utilization of protein in SBX has been reported to be limited due to high temperature and high shear conditions. Supercritical fluid extrusion (SCFX) uses supercritical CO2 (SC-CO2) as a blowing agent, enabling utilization of heat and shear sensitive ingredients including whey protein. The objective of this study was to investigate the effects of whey protein and process parameters on the expansion and microstructure of SCFX extrudates using on-line slit die rheometer, X-ray microtomography, and video analysis. The potential of SCFX for healthy snack production with whey protein and with higher resistant starch (RS) content was also evaluated. Whey protein acted as a diluent leading to reduced melt viscosity that resulted in lower cell number density and overall expansion. Thermal analysis indicated only limited chemical interaction between whey protein and pregelatinized corn starch. SC-CO2 increased the expansion of whey protein added starch-based extrudates but structural collapse was observed at 0.75 wt% SC-CO2 level during post-extrusion drying at 85oC. Cell size from transverse cross-sections of SCFX extrudates decreased with radial distance from the center. In the longitudinal direction, the cells shapes were more elliptical than spherical. Not only piece density but also the ratio of cell wall thickness to cell diameter were observed to be a good predictor of mechanical properties. SCFX process showed 30-200 fold longer expansion time compared to SBX and can be utilized for making novel products. The pressure drop profile in the die was found to be critical in controlling not only overall expansion but also the rate of expansion. SCFX extrudates showed 4 fold higher RS content than SBX extrudates while the product densities of both extrudates were comparable. Feed moisture content and processing temperature were found to be critical for RS formation in SCFX and SBX extrudates. A SCFX process has been successfully developed for production of novel healthy snack containing 40~60 wt% whey protein with unique porous structure and texture. The finding showed that an extrusion process at temperatures below protein denaturation temperature using SC-CO2 can help prevent hard texture and create a uniform microstructure.