Recent advances in gabion construction techniques have increased the interest for gabion stepped spillways. The present study investigates the air entrainment process in the stepped cavity of a gabion stepped spillway through physical modelling. The microscopic air-water flow characteristics were investigated using a phase-detection probe, complemented by some video observations. The visual observations indicated a high level of interactions between stepped cavity flow and gabion seepage, including some bubbly flow motion in the gabions. The measurements showed relatively lower aeration, velocity and turbulence levels than that in the mainstream flow. The air-water measurements demonstrated some self-similarity of main flow properties above the pseudo-bottom formed by the step edges at all measured locations.