An experimental study has been conducted in a Mach 2.5 flow to understand the shock-wave boundary layer interaction (SWBLI) over a forward-facing step (FFS) of step height equals twice the oncoming boundary layer thickness. Particle Image Velocimetry (PIV) is the primary methodology used to study the unsteady shock oscillation phenomena. Prior studies show shock oscillates one or two orders smaller than the characteristics frequency of the incoming boundary layer. Particle Image Velocimetry has done both in cross-stream plane and spanwise plane. Instantaneous PIV images in the cross-stream plane shows shock foot oscillates over a distance of approximately one step height. From PIV study in the cross-stream plane, it is observed that the incoming boundary layer velocity fluctuations (upstream parameter) is corelated to the shock foot oscillation and the separation bubble (downstream parameter) is correlated to the shock motion for the forward-facing step of step height equals twice the incoming boundary thickness. © 2021, Springer Nature Singapore Pte Ltd.