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Shear thickening fluid (STF) impregnated fabrics offer improved ballistic performance against impacts, but the effect becomes not obvious at high impact velocity (e.g. 300 m/s) as reported by some. This paper presents findings from an investigation of STF-impregnated fabric panels against ballistic impact, and attempts to identify the failure mechanisms of such panels under high velocity impact, which will shed light on effects of STF-impregnated fabrics for low and high velocity impacts. Single-ply and 10-ply neat and STF-impregnated aramid fabric panels were experimented on at impact velocities around 500 m/s. The results indicated that the specific energy absorption of the single-ply and 10-ply STF impregnated fabric panels was 44.8% and 64.1% lower than that of their neat counterparts respectively. The mechanisms were studied theoretically and morphologically. It was found that the projectile velocities perforating the fabrics were decreased by STF impregnation due to the total movement constraint of the primary yarns. This changes the failure mode from tensile dominant to shear dominant, increases the possibility of earlier damage and failure of the primary yarns, and reduces the pull out distance, causing decrease in the energy absorption. The findings are significant for guiding further design of STF impregnated fabric panels for ballistic protection. [ABSTRACT FROM AUTHOR]