As an advanced structural ceramic, boron carbide is drawing more attention as a promising material with its low density and excellent mechanical strength. Granular flow is triggered by progressive fracture of boron carbide during high strain rate (104 – 105s-1) and high pressure (several GPa) impact, and has critical effects on ceramic performance. However, granular flow at such high rates has not been well characterized. To understand dynamic granular flow under different levels of pressure and strain rate, we designed and performed multi-axial loading experiments of pressure shear plate impact on a commercially available boron carbide powder with an average grain size of 0.7 micron. The granular boron carbide powder is deformed at a shearing rate as high as 1×105 s-1 with a superimposed hydrostatic pressure up to 3.1 GPa. The granular flow shear stress is ~450MPa under such pressure and shear strain rate. From all plate impact shots, shear strength of such granular material increases with increasing pressure, showing a strong pressure effect and having been confirmed by other kinds of experiments and simulations. Post-mortem TEM characterization provides useful insights of mechanisms such as grain fracture or amorphization during the deformation.