The critical current density J c under hydrostatic pressure to 0.6 GPa is determined for melt-textured YBa 2 Cu 3 O x (6.4 \leqslant x \leqslant6.9) rings containing single [001]-tilt grain boundaries (GBs) with artificial mismatch angles. We found that J c increases rapidly under pressure at the rate 20 to 40% GPa -1 . Such a large enhancement cannot be explained by a simple tunneling model where the GB width is compressed under pressure. The intrinsic pressure dependence of J c is estimated from the temperature pressure hysteresis to be 15 to 25% GPa -1 , revealing a substantial relaxation effect in the GB. Both intrinsic and relaxation effects make important contributions to the large pressure dependence of J c observed in the present experiments. YBa 2 Cu 3 O x bicrystalline rings generally have vacant oxygen sites in the GB region. Filling such sites with oxygen anions should lead to further enhancement in J c .