The article examines the Mach-number effects on vortex breakdown in subsonic flows over delta wings. The governing equations are three-dimensional compressible Navier-Stokes equations. The length and velocity are non-dimensionalized by the chord length and sound speed of the free stream condition, respectively. The computational code used here is based on the well validated Navier-Stokes code with recent modifications to realize more efficient implicit time-integration schemes and a high-order accurate evaluation of space derivatives. The wing-surface contours show pressure distribution is observed for the cases with a Mach number greater than 0.5. Here, the mode is distinguished by the instantaneous shape of the iso-surface of the second invariant of the velocity-gradient tensor. First, the results of the implicit large-eddy simulation and Reynolds averaged Navier-Stokes hybrid simulation using the Baldwin-Lomax turbulence model with a Mach number of 0.065 were validated with the experimental study.